Improve cross python compatibility issues with little imports
Add index to AccessToken.token

Without an index, the retrieval of the access tokens performs a full table scan, and after just a few couple of thousands tokens it will significantly affect the performance.
Minor formating tweak to last README change.
Removing need to client_secret during password grants for public clients

As discussed in issues #21 and #25. This provides a more rigerous check
of the circumstances required for the client_secret to forgone.
Not sending a refresh_token to public clients requesting password_grants
Public and private clients can now have different expiry times

Public clients often wish to have shorter expieration times
Clean up and PEP8 for #41.
Bumping version to 0.2.7-dev to avoid ambiguity in an established package.
Addressing #28 by replacing the BSD license reference with MIT to match what's in the repo.
#29 - Remove trailing slash that was causing installs to fail on Windows.
Fixes #32 - Add `token_type` to access token response to conform to section 4.2.2 of the OAuth 2.0 specification.
Adding a License section to README.