Add ParamType

I've filtered down the possible types of parameters that a Command can
have to one of three:

- no parameters
- a String parameter
- a Map<String, Slide[]> parameter

This enum will be passed to the Message constructor and used by the
Command interface when I add methods to retrieve message payload.
Unsupported parameter types will probably either return null or throw
UnsupportedOperationExceptions.
Integrate Message with ParamType
Add payload retrieval methods to Command

With ParamType and its standardization of Message parameter types, I can
now create a coherent, interface-level way of retrieving the parameters
to a given Message. Polymorphism FTW!