Merge pull request #50 from mark-adams/fix_tests_when_deps_missing

Fixed tests so that they will still run without failure when dependencies (PyCrypto, ecdsa) are missing
Modified RSA algorithms to use cryptography library instead of PyCrypto
Modified ECDSA algorithms to use cryptography library instead of ecdsa.
Added some missing ensure_bytes in a couple of the tests.
Consolidated testing variables for has_rsa and has_ecdsa into has_crypto.
Updated README and dependencies in tox.ini