Add script to update libgit2

A call to ".\UpdateLibgit2ToSha.ps1 deadbeef" will:
- Fetch libgit2
- Checkout the sha, if it exists as a commit
- Build 32- and 64-bit variants of the library, and copy them to
NativeBinaries
- Update the libgit2_sha.txt file
Add a build status section to the README file
Switch README content to reference-style linking
Add optional RespositoryOptions to Repository.Init call

Partially fix #263
Add optional RepositoryOptions to Repository.Clone call

Fix #263
Add .mailmap file
Update copyright notice for 2013
Eagerly load the Configuration if any config path is provided

Fix #277