Introduce repo.Index.IsFullyMerged
Introduce IndexEntry.StageLevel
Fix IndexEntry IEquatable implementation
Minor code refactoring
Fix some issues pinpointed by Code Analysis
Ensure Diff.Compare() cannot apply on a bare repository
Enhance Index.Unstage test coverage
Make Index.Unstage() work against an orphaned Head

Fix #257
Enhance Index.Unstage() code coverage
Index.Unstage should accept absolute file paths
Make Diff.Compare() able to compare a null Tree against the Index and Workdir
Enhance Diff.Compare() test coverage
Simplify Index.Unstage() implementation
Enhance Index.[Stage|Unstage]() test coverage
Adds Reset(ResetOptions resetOptions, Commit commit) method in Repository
Adds Reset(Commit commit, IEnumerable<string> paths = null) to Repository
Move reset overloads to extension methods
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