Clean up state tests

This removes top-level vars in favor of describe-scoped vars.
Prevent multiple copies of express-state from overwriting `expose()`

If more than one copy of express-state is loaded, because of dependency
issues, it will not plugin into the same copy of express more than once.
Update change history for 0.0.2 and prevent multiple copies change
Move multiple copies tests to their own file

This moves the express mock and dynamic `require()` loading via mockery
into its own test file.
Merge pull request #4 from yahoo/multi-protect

Prevent multiple copies of express-state from overwriting `expose()`