Add update method for Account

[danfowler]

• Add input/output param docs for method
• Add test
• Add update method

[rezigned]

I think we should re-write these a bit to make it clearer.

self.assertFalse(account.chain_enabled)
account.update(chain_enabled=True)
self.assertTrue(account.chain_enabled)

Otherwise it looks like we just assign object's attribute directly (even though there's more stuff under the hood).