enable creation of multiple elements

add a list of faces or vertices to a mesh
Merge pull request #15 from pearswj/9-multi-add

Adding multiple elements
add failing test for edge collapse #17

test case provided by @indigo99
improve existing edge collapse test cases
add multiple test cases to new collapse test
remove redundant code from edge collapse
fixes #17
further improve new edge-collapse test
Merge pull request #19 from pearswj/17-fix-collapse-edge

Fix #17 (valence 2 edge collapse bug)