sign_rsa_sha1 no longer requires pycrypto (tests pass)

-- NOT READY FOR MERGE --

This PR is not yet ready to be merged, and is just to track work towards ticket #226.


- [x] Implement `sign_rsa_sha1(base_string, rsa_private_key)` with tlslite
- [ ] Implement `verify_rsa_sha1(request, rsa_public_key)` with tlslite
- [ ] Swap pycrypto for tlslite in documentation references
- [x] Swap pycrypto for tlslite in setup.py