Rename low level reference creation methods

The idea is to keep exactly the same name as in libgit2
Merge branch 'master' into next
Implement the Repository mapping interface in Python

Add 'Repository.get' and low level 'Repository.git_object_lookup_prefix'
Support short (raw) oids
Merge branch 'master' into next

Conflicts:
	src/repository.c
Merge branch 'master' into next

Conflicts:
	include/pygit2/types.h
	src/oid.c
Fix for Python 3
Merge branch 'master' into next
Fix py_str_to_git_oid for raw oids