Fixes #275 - No longer warn of unnecessary comparison with -Wextra

-Wall -Wextra previously generated the following:
   src/libwebvtt/node.c:164: warning: comparison of unsigned expression >= 0 is
   always true

Removing the comparison '>= WEBVTT_NODE_INTERNAL_START' fixes this warning.

I've also made these macros more in-line with the style guide.
Merge pull request #275 from caitp/issue-275

signed/unsigned warning on WEBVTT_IS_VALID_INTERNAL_NODE