Tests create global config stub if required

Instead of relying on the global configuration and skipping the test if
Git is not installed, the following tests create a configuration file
containing user.name and user.email using
BaseFixture.CreateConfigurationWithDummyUser(Signature):

* CommitFixture.CanCommitWithSignatureFromConfig()
* ConfigurationFixture.CanEnumerateGlobalConfig()

Fixes #561
Enforce Network.ListReferences() test coverage

This puts under the light that DirectReferences may point to GitObjects unknown from the local repository.
Improve Network.ListReferences() documentation
Update libgit2 binaries to 98eaf39

https://github.com/libgit2/libgit2/compare/e87d9d3...98eaf39
First stab at blame marshalling
Spike Repository.Blame
Fix marshalling structures
Avoid stealing ownership of signatures
Hunk by-line accessors
Another marshalling test
Don't use nested classes
Satisfy MetaFixture
Dispose of native blame handle asap
Newlines at end of new files
Expose Blame on IRepository
Move blame flag mapping to extension method
Use LaxUtf8Marshaler
BlameStrategy.Normal -> Default
Ifs should have braces
Move blame options to external class