Fix name-mangling bug in compiled SubDomains only visible with clang compiler.
Replace Armadillo with Eigen3.

Eigen3 provides a more extensive interface, and support for sparse
linear algebra. Primary motivations behind the change are:

1. Eigen is header-only, which simplifies build and linking

2. Eigen allows row-major storage (unlike Armadillo), which is useful
when interfacing with parts of DOLFIN and other codes.
Fix demo data downloads on osx (curl command was broken).
Fix auto detection of PETSc/SLEPc 3.4.2 on Debian.
Merge branch 'garth/eigen'
Update README.rst with new links.
Detection of the RT Library does not make sense on OSX.
Add MeshQuality class to help with quantifying mesh quality.

Includes support for making histograms for Matplotlib.
Add MeshQuality to tests.
Handle case that ratio = 1.0.

Pointed out by Chris Richardson.
Merge branch 'fix-issue-125'
Remove Armadillo unit test.
Unit test updates for mesh quality computattion.
More unit test fixes.
Merge branch 'garth/eigen'
Merge branch 'garth/mesh-quality-stats'
Improve detection of Python module path.

Fix is working towards good MacPorts support.
Work on proper discovery of Python paths.