Add String binding construct that takes a char * array and len.

- I've added a String binding constructor that takes a char * array and a default len
of -1. This way we can just pass literals into the constructors which will simplify
a lot.
- The String binding constructor that takes a byte * array also accepts a default len
of -1 now.
- webvtt_create_string_with_text now finds the len of the byte * array that it is
passed if the len it is passed is -1.
Merge pull request #177 from RickEyre/issue-167-squash

Add String binding construct that takes a char * array and len.
Merge pull request #178 from caitp/issue-178

Remove define at include/webvtt/util.h:176
Fixes #175 -- Minor API change, remove uninitialized variable

Bindings have been updated to support appending strings without
providing a length parameter (to support string literals)

In addition, append now has overloaded functions which provide a
webvtt_status out-parameter to allow us to determine if an error has
occurred when using the bindings.