Add new elastity demo (Python) using smoothed aggregation AMG.
Add pulley mesh to download list.
Add first rough (but working) version of c++ elasticity + AMG demo
Add C++ smoothed aggregation AMG demo.
Switch default serial LU solver when using 64-bit indices.

This follows a bug fix in PETSc.
Add missing UFL file for elasticity AMG demo
Add unit test for smoothed aggregation AMG.
Minor tweaks to new AMG demo.
Improve smoothed aggregation AMG test and update ChangeLog.
Merge branch 'master' into garth/fix-issue-430-samg

Conflicts:
	ChangeLog
Merge branch 'garth/fix-issue-430-samg'

Conflicts:
	ChangeLog
Merge branch 'garth/switch-default-lu-solver-64-bit'
Use Lawrence Mitchell's suggestion to get branch name in cmake.local script.