Auto detect Client interface to use on some boards

Only boards that have a default, intrinsic way of handling
networking (like WiFi boards, such as MKR1000) are recognized.
Also, only boards we could test or that have well-known
implementation and good documentation are recognized.

Auto detect Client interface to use on some boards

Only boards that have a default, intrinsic way of handling
networking (like WiFi boards, such as MKR1000) are recognized.
Also, only boards we could test or that have well-known
implementation and good documentation are recognized.

Support Arduino CI (#31)

Also fix a few minor bugs.

Fix message crackers (#30)

Wasn't handling JSON objects and "solo" numbers correctly.

Do not await disconnect forever (#29)

There was a bug in PubSubClient::stop() - a surplus call to
`input_state()`. Also, it now waits for data shorter, not the usual
"full (default) time" . This time is not yet user settable.

Since ESP8266 can, in some weird and as-of-yet-unexplained situations,
take a _long_ time (hours, even) to change the state of the
`WiFiClient` to "not connected" after a `stop()`, we no longer await
this disconnection "forever".  There is now a timeout (for now, not
user-settable) and we give up after a short while. This might lead to
some (probably temporary) leaks, but, we haven't run into problems in
our testing.

Don't wait for disconnect in a tight loop (#27)

Instead, do a short `delay()` in each iteration. This should
allow the rest of the SW to "do its stuff", like, kicking the
WDT and such.

Remove retries and enable setting TCP/IP port (#26)

Retries were not limited, making it possible to have a
_long_ blocking call, which is not a nice thing in Arduino
`loop()`. Limitting would only partially fix the problem.
Removing them altogether works much better and user can
retry at her own convenience.

Support for setting the TCP/IP port was added, which is needed
when using a TLS/SSL (network) client class. Before, we always
used HTTP port, which would not work.

Improve docs and fix example typos

Improve documentation (fix typos, improve explanations...) and fix typos in examples.

Support for ESP32 and 'message crackers'

The `WiFiClient` of ESP32 has non-standard behavior. When the
(TCP/IP) connection is closed, it will start returning 0 as the result
of `available()`, even though not all octets were read, and one can
actually still `read()` them. So, we need "a middle-man" to keep track
of how many octets are actually available for reading.

This is, unfortunately, a slightly incompatible change. You need to
use the new class `PubNonSubClient` for this to work.  So, even though
you don't _have_ to (and then ESP32 will not work, but it didn't work
either in v2), it is different and warrants a major version bump.

Also, for ease of use, we introduced "message crackers", which should
inspire users to avoid the old `Client` based interface, making it
easier to do similar upgrades/changes in the future.

Examples were also cleaned-up:

* Use new high-level API (message crackers)
* Use ArduinoJson library in the Josn example
* Update comments
* Remove not-very-useful examples
* Reformat with Clang format

Develop (#18)

* Improve debug logging.

* Version bump