Shorten projection tests

Use smaller meshes so the convergence tests take less time.
Implement projection of Expressions into vector-valued function spaces

For spaces that don't support PointEvaluation, we first interpolate
into CG space of appropriate degree and then project into the target
space.
Add more tests of project functionality
Merge pull request #42 from firedrakeproject/projection-updates

Support projecting expressions into vector value function spaces.
Additionally, provide useful error messages in the case of
bad input.
Linkify github in obtaining firedrake docs
Fix of doc string for ExtrudedMeshes.
Make helmoltz and project extrusion tests shorter.
Fix docstring for make_extruded_coords
Merge pull request #43 from firedrakeproject/extrusion

Shorten extrusion Helmholtz / Project tests, docstring fixes.