Remove encoding/json dependency

The only purpose of using the built-in Go was to encode json
strings that had unicode or needed to escaped.

This commit adds the new function `AppendJSONString` which allows
for appending strings as their json representation to a byte
slice.

It's about 2x faster than using json.Marshal.

Update SYNTAX.md

Add tostr and fromstr modifiers

For wrapping and unwrapping json strings

Set array index as key for ForEach

See tidwall#248

Fix modifier bug in multipath selector

Closes tidwall#253

Added Path and Paths for getting the original path of a Result

This commit adds a two new functions of the Result type:

- Result.Path:  Returns the original path of a `Result` that was
                returned from a simple `Get` operation.
- Result.Paths: Returns the original paths of a `Result` that was
                returned from a `Get` operation with a query.

See issue tidwall#206 for more details

Fix empty string operator not matching

fixes tidwall#246

cleanup test

Added @keys and @values modifiers

The "@keys" and "@values" modifiers converts an object into an
array of its keys or values respectively.

Take this json for example:

{"first":"Tom","last":"Smith"}

@keys   -> ["first","last"]
@values -> ["Tom","Smith"]

This feature was requested in tidwall#161.

The Array method should return back one item for JSON objects.

This commit fixes an issue where the Array method was not
returning single value arrays when the reciever Result was a
JSON Object.

fixes tidwall#240