[Python] improve and standardize simple_endpoint.py

[suntzu86]

RELATED TO: #279

In simple_endpoint.py, we provide a few wrappers around MOE REST endpoints. At the moment, they're pretty poorly documented. With 3 (at the time of this writing) functions that require fairly similar inputs, the function interfaces are all a little different. Some take an Experiment, some take list of points, some use HistoricalData, etc. Some wrappers overwrite input data differently than others, etc.

If we're going to provide these simple wrappers, we should standardize them and increase DRYness.