Update README
Split the Refspec type to its own file
docs: fix warnings
Update copyright year
utils: allow borrowing a C string

We don't always need our own copy of a C string; sometimes we just need
to parse it. Create py_str_borrow_c_str() which returns a char pointer
to python's internal value string, with which we can avoid an extra copy.
config: borrow the string for lookup and setting

We don't need our own copy of the string, so use the new borrowing
mechanism to use python's underlying string for the key to get/set.
repository: borrow C strings where possible
signature: borrow the name's C string
refspec: borrow the C string
remote: borrow the C string where possible
index entry: avoid extra copy
Get ready to release 0.20.2
walk: make the sorting mode optional

Since the libgit2 has a default sorting method, which we also mention as
default in the documentation, there is no particular need to make the
user choose a sorting method when the order does not matter. We use
sorting NONE in that case.
Fixed `undefined symbol: PyLong_AsSize_t` on PyPy
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