[CORE-0.3] Serialize classes so that they are accessible from all services

[blythed]

Currently, if a user creates his own custom Component:

class MyComponent(Component):
    ...

then, this code needs to be accessible in the installation of all environments using MyComponent. It would be good to remove this requirement. For instance, the source code code be serialized in the db.metadata, along with the parameters.

Possible solution

1. Serialize class with dill
2. Serialize the __repr__ of the class, to give users something to see
3. Add this serialization to metadata store with _BaseEncodable and base64 encoding
4. Reload the class definition to an in-memory module
5. Save the parameters and artifacts as normal