Add ability to invoke CoinbaseAccount with a known good oauth access token

[skruger]

This pull request implements setting an arbitrary access_token for use when an oauth implementation whose data structures are not compatible with what the oauth2_credentials parameter expects.

In my testing I also ran into a situation when I was working with a string that was cast to unicode that produced an error when doing the 'type() is str' test. I modified the two instances of this (one in my new code and one in existing code) to check if the variables were instances of basestring instead.

[sibblegp]

Can you pull the latest version of the library and make this function?