added write method
Add Odb hash function.
Test for Odb hash function.
Remove unnecessary cast.
Add Is$Type methods to Reference.

This patch adds the following methods to Reference:

	IsBranch() bool
	IsRemote() bool
	IsTag() bool

which correspond to the `git_reference_is_$type` functions in libgit2.
Merge commit 'refs/pull/53/head' of github.com:libgit2/git2go

On top: fix git_buf handling and rename signature

This fixes #57, #54.

Conflicts:
	git.go
	reference.go
	repository.go
	submodule.go
Merge pull request #55 from lye/add-is-x-reference-methods

Add Is$Type methods to Reference.
Merge pull request #51 from Tobscher/master

added write method to index
Merge pull request #52 from ursachec/master

Support for git_odb_hash.
Return SubmoduleRecurse
add custom refdb backend support
Merge branch 'master' into custom_odb

Conflicts:
	git.go
	reference.go
	repository.go
	submodule.go