Improve best practices in contract code

[matthewjablack]

Description

This PR improves contracts to follow best practices.

Submission Checklist :pencil:

• [x] QSP BP 5. Ensure Oraclize.sol constructor checks that parameters are non-zero
• [x] QSP BP 13. Add MedianizerInterface.sol to ensure that cyclic import does not occur between Medianizer.sol and Oracle.sol.
• [x] QSP BP 15. Label functions without explicit modifier as public
• [x] QSP BP 16. Change token transfer in Medianizer.fund to use uint256 instead of uint
• [x] QSP BP 20. Add comment to Oracle.sol to ensure uninitialized constant Medianizer med is initialized in child constructor
• [x] QSP BP 22. Oracle.setAssetPrice has extremely long line which is split into multiple lines
• [x] QSP BP 24. Fix inconsistent function parameter names to always use underscore except with queryId
• [x] QSP BP 26. Add reason for error in require statements