Use an empty tuple as key if passed in no bcs
Docs: fix broken contributing link on contact page
Allow passing single DirichletBC when solving linear system
Use PyOP2 object caching for function spaces

Rather than reimplementing the same scheme, just let FunctionSpaceBase
inherit from ObjectCached.
Only try and delete petsc options if set up

If we get an error building a NonlinearVariationalSolver object before
we've set the options prefix, then we should not try and look at it in
__del__.
Merge pull request #239 from firedrakeproject/fix/map_cache

Minor fixes for new PyOP2 caching model.
Add requirements file
Travis: install from local requirements.txt
Docs: add installation instructions for FFC, FIAT, UFL
Merge pull request #238 from firedrakeproject/requirements

Require FFC, FIAT, UFL as dependencies
DMPlex: Turn dmplex.py into and extension module with PETSc linkage
DMPlex: Optimise local facet numbering

Local facet numbering is now performed via the PETSc C-interface.
The vertex numbering required for this is also generated in C and
only computed once during Mesh initialisation.