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.
Add function GenericMatrix::nnz() to get number of non-zeros in matrix.

Fixes Issue #110
Update ChangeLog
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.
Add back missing sed in cmake.local script.
Updates to AUTHORS file.