Claim last bit for FileStatus.Nonexistent

Fix #307

Also, remove comment reference to GIT_STATUS_NOTFOUND which, ironically, does not exist
Rename CHANGELOG.md to CHANGES.md
Include third-party product name in license filename
Add libgit2 license
Update NuGet package specification

 - Include libgit2 licence
 - Filters out the *.ilk and *.lib files while copying the binaries.
Introduce repo.Network.FetchHeads
Update libgit2 binaries to 5f9f69d

https://github.com/libgit2/libgit2/compare/586f71e...5f9f69d
Enhance Configuration test coverage

Fix git-tfs/git-tfs#300
Add build.libgit2sharp.x64.cmd
Check all [Flags] enums for overlaps

Closes #311
Initial push implementation
Fix AheadBy/BehindBy behavior for orphaned branch
Teach BranchCollection to disambiguate local and remote branch names
Make Diff.Compare able to expose type changes

Fix #196
Prevent test cleanup from hiding potential test failure
Make repo.Fetch() accept a Credentials instance
Remove trailing whitespaces.
Remove leading tabs.
Fix filemode for source files.
Add missing Network property to IRepository interface