[Python] Standardize json serialization and deserialization of MOE classes

[suntzu86]

This follows a comment I made in #241 which was out of scope for the that pull request. This issue is also touched on in #34.

The current proposition is:

1. Define a "JsonableInterface" that supports something like to_json() and from_json() (static?) methods (the latter of which is a factory method). The idea then is that then users can:

   foo_object = Foo(...)  # regular way
   # or
   foo_object2 = Foo.from_json(valid_foo_object.to_json()) # maybe **valid_foo_object.to_json()
   

   In simple cases, we can just set from_json = __init__.

2. Any object that should be json-able should implement these two functions.

   Superclasses can implement baseline methods and subclasses can extend the resulting dicts and/or access extra json members in construction.

3. Longer term, it'd be nice to accomplish the general goal of #34 which is to hook our colander schema deserialize/serialize functions up to the MOE classes directly. That is, we override colander's serialize/deserialize methods to call the to/from_json() methods of the object that the schema represents.

   This may also let us push validation into objects (instead of having objects and schemas validate separately right now).