Removed some unnecessary references to PyCrypto's Crypto.Hash functions in the ECDSA code since it uses hashlib functions anyways. This removes an ECDSA dependency on PyCrypto.
Fixed tests so that they will still run without failure when
dependencies (PyCrypto, ecdsa) are missing. (Tests are skipped if
libraries are not present.)
Added reference to unittest2 so that Python 2.6 can use skipIf
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