Create new abstract base class for LocalAccount

[carver]

With the incoming work on #25 (in PR #26) -- it's time to have an abstract class defining the interface for a LocalAccount, plus some docs about which methods must be implemented. Specifically:

• address property is required
• signHash() is required
• signTransaction() is required
• __hash__() and __eq__() are required - so care should be taken that the account cannot be mutated to use a different private key.
• privateKey property is optional, since some sources refuse to reveal it (like a hardware wallet)
• encrypt() is optional, since some accounts cannot reveal the private key

(Also, I just noticed that the docs for encrypt in LocalAccount are wrong. See the definition in Account instead