1. Moved oauth.py to __init__.py

2. Refactored __init__.py for compatibility with python 2.2 (Issue 59)

3. Refactored rsa.py for compatibility with python 2.2 (Issue 59)

4. Refactored OAuthRequest.from_token_and_callback since the callback url was
getting double url-encoding the callback url in place of single. (Issue 43)

5. Added build_signature_base_string method to rsa.py since it used the
implementation of this method from oauth.OAuthSignatureMethod_HMAC_SHA1 which
was incorrect since it enforced the presence of a consumer secret and a token
secret. Also, changed its super class from oauth.OAuthSignatureMethod_HMAC_SHA1
to oauth.OAuthSignatureMethod (Issue 64)

6. Refactored <OAuthRequest>.to_header method since it returned non-oauth params
as well which was incorrect. (Issue 31)