Add support for computing jacobians for quad extruded meshes in 2d and 3d.
Handle inverse jacobian computation for 2d and 3d quads.
Docs: Add PyOP2 concepts
Docs: Add code examples to concepts
Concept docs: expand the section on data
Concept docs: expand par_loop section
More on par_loops, access descriptors, direct loop example
Add par_loop examples using Const and Global
Expand the introduction and the section on sets
Use sphinx shorthand to strip package prefix
Add reference markers for every (sub)section
Merge pull request #298 from OP2/extrusion-quad-jacobi

Add jacobian computations for extruded quads (interval x interval).
Bump PyOP2 FFC compatibility version to 0.5.0
Bump version to 0.9.0
Merge pull request #295 from OP2/doc/concepts

Add user documentation on PyOP2 concepts
Add progress context manager to log utilities
Add compiling progress around host compilation
Merge pull request #302 from OP2/chatty-progress

Logging gains a with progress(...) context manager (arguments the same
as for log).  Use it to spew "I'm compiling something" messages at INFO
log level.
Add halo exchange functions on MixedDat

Previously we deferred to the superclass functions, which don't do the
right thing.
Create correct sparsity pattern for non-square blocks