Some python3 fixes
Install the "decl.h" file
Remote: make sure to take the contents of the id

Pass the contents of the buffer containing the git_oid to bytes() so
build a raw representation of its contents. Using bytes(ffi.buffer(...))
returns the representation of the buffer.
Depend on the cffi package

Let both pip and Travis know what we need.
Fix installation-time cffi compilation

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.
Safer repository pointer extraction

Casting a pointer to a non-pointer type is something which you should
never do. Instead, do something a bit more convoluted, but which is
guaranteed to give us the right pointer, as well as making sure that the
memory we exchange between python/cffi and the C extension is of the
right pointer size.

While we're touching this code, fix which object we pass to the Remote
constructor to keep alive. We need to pass the Repository object to
stop it from becoming unreferenced (and thus freeing memory the remote
needs) instead of the git_repository pointer.
ffi: style changes

Make to_str() accept None as well as ffi.NULL to return as a negative
value, and grab the version in a more compatible way.
Optionally skip online tests

It sometimes happens that one finds oneself on a train. Exporting the
PYGIT2_OFFLINE environment variables disables the tests that need a
working network connection for such cases.
Remote: protect against invalid input on rename

The C API expects a non-NULL new name and raises an assertion if we
don't protect against such input, so let's guard against falsy values,
which also takes care of the empty string, which is also not valid
input.