Implement MsgProposeBase

[clevinson]

• MsgProposeBase
  As a member of a group I want to create a new proposal for one of our group accounts so that the members can vote for it in order to
  execute it on success.

NOTE: this task can only be implemented as integration test as there are no other modules using the group module yet. This is considered an extension point

NOTE: I have created a new issue to address the execute now scenario and descoped it from this issue: #53

needed-by #31

[clevinson]

We should have some concrete examples with message payloads from Regen's use cases to verify the real use cases for this, and verify that authentication is sufficient.

Side note: Does the use case here have to do with community DAOs?