Upgrade to libgit@0.21.0
Remove deleted file
Add NULL params to git_repository_set_head
options instead of opts
id instead of oid
Add missing NULL params
Add missing files to binding.gyp
Ignore unused function warnings from libgit2
Add new Windows files
Merge pull request #30 from atom/ks-libgit2-zero-dot-twenty-one

Upgrade to libgit2 0.21.0