fix #21 Allow authenticated requests in tests

- Remove lazy getting behavior in getCurrentUser(); the rest object
  is not bound to each request, but to the server instance, so that
  would be a terrible plan.
- Only create new token for user at login time if they
  actually need one.
Add basic folder children test.

It's very incomplete, but at least it touches folder creation
and some basic access control stuff.
Improve folder test.
fixes #33. Add /version endpoint to API.

It will output the current semantic version of the API.
Merge pull request #34 from girder/folder-test

Add (minimal) folder test