First pass implementation of is_valid() for all Message types

[sambarnes]

The majority of the validation rules can be applied without the need for a built up blockchain state. These rules can be seen at a high level in Who's forum post, while the fine details should be double checked in factomd code.

• add is_valid(self) function stub to all Message classes
• implement validation rules that do not require state
• insert TODOs for rules that were skipped that do require state and create tickets for the work
• write unit tests