Add example for MDP/0.2.
Add buildable implementation of MDP/0.2.
Improve test.
Mock Process().
One successful req-rep cycle!
Add *Context.SetLogger().
Remove ContextIsNil, SocketIsNil, and NotImplemented.

panic() is more appropriate in these cases.
Improve SetLinger and Close.

Increase test coverage, deal with some error cases more correctly,
and include inline documentation about assumptions.