Bump libgit2 with breaking refs changes
Reference changes
Add Docs for Branch#head? and some other tiny cleanups.
wrap git_reference_is_valid_name as Rugged::Reference.valid_name?
Merge remote-tracking branch 'origin/refname-validity' into development

Conflicts:
	ext/rugged/rugged_reference.c
Clean `valid_name?`
Merge branch 'development' of git://github.com/libgit2/rugged into documentation

Conflicts:
	ext/rugged/rugged_reference.c
Fix docs for Reference.valid_name?.
Cleanup and extend Reference docs.