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.