Switch to master as default branch and cleanup gh-pages

[nelsonsilva]

Hi guys,

I just merged latest into master. I also switched to master as the default branch since we're getting closer to a Dart 1.0 release and there shouldn't be any major breaking changes. So, please start using master and we'll soon publish it to pub as well.

I will also delete the existing gh-pages branch and push a fresh one since it is currently a mess.

Also, please join our mailing list at https://groups.google.com/forum/#!forum/three-dart since in the future i'll post announcements there.

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