Lint tests on make lint
Remove all instances of from foo import *
Move from UFL import * first in __init__.py

We override some UFL classes with our own, so import UFL first such that
when the user imports Firedrake they always get ours where appropriate.
Only import appropriate parts of pyop2 in __init__
Add __all__ to all modules that are imported in __init__

To avoid polluting the firedrake namespace, ensure all modules only
export their public symbols.
Merge pull request #221 from firedrakeproject/fix/remove_star_imports

- Remove "from foo import *" from all firedrake modules.
- Add __all__ to all public firedrake modules to clean up namespace.