Gumbo: Integrate from GitHub.  This syncs up to https://github.com/google/gumbo-parser/commit/17d5f7b6789b89506f78ce2d6126eac75204aaab

Commits included in this CL:

* Change naming convention for struct tags from _Gumbo to GumboInternal so it doesn't violate C reserved word conventions.
https://github.com/google/gumbo-parser/commit/1e892b1a1d98c4c85237010f0cf44e441aefa0ee

Also added the .gitmodules, CONTRIBUTING.md, autogen.sh, and gumbo_parser.gyp files, which have been contributed by outside authors but didn't make it into the last integration.
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Created by MOE: http://code.google.com/p/moe-java
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