test: clean up

Remove the temporary files and directories we create as part of running
the test suite.
object: implement peel()

This simplifies getting one type of object from a higher-level one as
well as accepting a commit-ish or tree-ish.
object: allow passing a type object to peel()

Allow usage of a python type instead of having to use the libgit2
constants.
Merge remote-tracking branch 'carlos/object-peel'
docs: clarify git-init recipe

Fixes #336
docs: document TreeBuilder.get

Fixes #302