Rename "quadrature" measure --> "quadrature_cell"
Add measure type "quadrature_facet"
Small bugfix in Measure.__str__
Import dL into global namespace
Replace quadrature_cell and quadrature_facet measures by custom measure
Merge branch 'master' into logg/topic-multimesh

Conflicts:
	ufl/algorithms/change_to_reference.py
	ufl/algorithms/printing.py
	ufl/measure.py
Small fix in unit test for custom integral