Our error handling in Algorithm.prepare_key() was previously weird and
kind of inconsistent. This change makes a number of improvements:

* Refactors RSA and ECDSA prepare_key() methods to reduce nesting and
  make the code simpler to understand
* All calls to Algorithm.prepare_key() return InvalidKeyError (or a
  subclass) or a valid key instance.
* Created a new InvalidAsymmetricKeyError class that is used to provide
  a standard message when an invalid RSA or ECDSA key is used.

Cryptography previously lacked support for ECDSA keys in RFC 4253
format. Now that they have support for those keys, we should take
advantage of it and support them in PyJWT.

Implements #243.

The functions `decode_rfc6979_signature` and `encode_rfc6979_signature`
were deprecated in cryptography 1.0:
    https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst#10---2015-08-12
and raise a DeprecationWarning now. The replacements are exactly the same.