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.
Merge pull request #175 from caitp/issue-175

Update webvtt_string_append to use a signed integer for length that uses strlen if negative.