Fixes #178 - Remove #define WEBVTT_SUCCESS() that hides webvtt_status

Removed `#define WEBVTT_SUCCESS( status )` from include/webvtt/util.h due to
the fact that it shadows the enum value WEBVTT_SUCCESS declared earlier in the
file.

Rueplaced uses of `WEBVTT_SUCCESS( status )` with `!WEBVTT_FAILED( status )`
throughout the library (only one occurrence in parser.c and 2 in string.c)
Merge pull request #178 from caitp/issue-178

Remove define at include/webvtt/util.h:176