Remove unnecessary None initialization
Fix response caching and response representation

Let account = coinbase.Account( … ).
Doing something like str(account.accounts) should implicitly retrieve
from the API, but merely doing account.accounts should NOT — otherwise,
doing account.accounts[ … ] will send two requests to the server when
only one is intended. Therefore, caching functionality is implemented
so that implicit requesting through str(), parse(), or repr() will
trigger conditionally.
Rename Account class to Client class
Create representational classes for account

 - Organize classes in modules
        - Move REST classes to separate module
 - Created models for interacting with Account (incomplete)