Fix response schema of the "getaccountstate" RPC method

[dethos]

What current issue(s) does this address, or what feature is it adding?
This pull request tries to fix #713

How did you solve this problem?
Changed the JSON serialization of the AccountState instances

How did you make sure your solution works?
Tested manually and updated the automatic test

Are there any special changes in the code that we should be aware of?
No, I do think I fixed it everywhere, but something might be missing.

Please check the following, if applicable:

• Did you add any tests?
• Did you run make lint?
• Did you run make test?
• Are you making a PR to a feature branch or development rather than master?
• Did you add an entry to CHANGELOG.rst? (if not, please do)

[coveralls]

Coverage increased (+0.003%) to 83.034% when pulling b43536c on dethos:fix-get-account-state into edf756d on CityOfZion:development.

[ixje]

Thanks! 👍