The documentation recommends adding the ffi code as an extension so it
gets built at the right time.
Make use of the LIBGIT2 environment variable to build and link the ffi
module the same way the C extension does so the user doesn't have to
export CFLAGS.
This moves enough code into python with CFFI to pass the test_remotes
unit tests. There is no credentials support yet.
There is a small change in the return value of Remote.fetch() in that we
now return a TransferProgress object instead of extracting a few values
into a dictionary.
