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.
Return None when converting a "Real" element to FIAT

Change check for cell orientation flag to continue if the fiat element
we get is None.
Implement Constant

Add a version of Constant (with data carried by a PyOP2 Global) that
behaves like an appropriate Coefficient to UFL and an appropriate
Function to Firedrake's assemble calls.

Effectively we build a FiniteElement, VectorElement, or TensorElement
for UFL and fake up enough Function methods that assembly needs no
changes to pass Constants in to the par_loop.
Add some tests of Constant
Add section on Constant in manual
Fix linking in manual sections