Remove pyca/cryptography backend's dependency on python-ecdsa

[mattsb42-aws]

Previously, the pyca/cryptography backend was dependent on python-ecdsa to handle the conversion between DER encoded signatures and the raw signature encoding that JOSE uses.

This fixes that, using the DSS encoding/decoding utilities provided in pyca/cryptography and adding simplified raw signature encoding and decoding directly.

Also added in here are modifications to the tox and Travis CI configurations to remove python-ecdsa and python-rsa when testing the pyca/cryptography backend in isolation. This is a workaround until we can determine how best to remove those dependencies when the "cryptography" extras is selected.

NOTE: The algorithms/test_RSA.py::test_cryptography_RSA_key_instance test in the "compatibility" test runs will still fail. That is addressed separately in #116. The "base" test runs also still fail, as discussed in #114.

[mpdavis]

LGTM