Add support for log file compression (natefinch#43)

* Check test file content, not just length.

It is insufficient to just check the length of test files,
especially given that many of the tests result in multiple files
that have the same content/length. Instead, actually check that
the file content is what it is expected to be. Vary the content
that is being written so that the test failures become apparent.

This also fixes a case where the length of the wrong value is
checked following a write (it happens to work since the length
of the value checked is the same as that written).

* Make timeFromName actually return a time.

Simplify the timeFromName parsing (we only need to slice once,
not twice) and actually parse the extracted time in the
timeFromName function rather than returning an abitrary string
that may or may not be a time. Also conver the timeFromName
tests into table driven tests.

* Add support for compressing log files.

Rather than scanning for old log files (under lock) when a rotation
occurs, a goroutine is started when we first open or create a log
file. Post-rotation compression (if enabled) and removal of stale
log files is now designated to this goroutine.

Scanning, removal and compression are run in the same goroutine in
order to minimise background disk I/O, with removals being processed
prior to compression in order to free up disk space.

This results in a small change in existing behaviour - previously
only logs would be removed when the first rotation occurs, whereas
now logs will potentially be removed when logging first starts.

* Rework file ownership test.

Previously the test only verified that the code called Chown
but failed to verify what it actually called Chown on. This
reworks the code so that we have a fake file system that tracks
file ownership.

This also simplifies upcoming additional tests.

* Clone file owner and mode on compressed log.

Clone the log file owner and the log file mode to the compressed
log file. Add tests to ensure that this is handled correctly.

update docs w/ backup format info

Update .travis.yml

had to update travis config since it doesn't use go get -t, so it doesn't get dependencies that only exist in test files.