ENH: Adding new extension MeshToLabelMap

MeshToLabelMap computes a label map from a model. Slicer contains a module called "Model to labelmap" module that performs the same operation but that has some limitations due to the asumptions it makes. For more information about the available (and upcoming) modules to perform a model to label map conversion, see discussion [1].

[1] http://massmail.spl.harvard.edu/public-archives/slicer-devel/2014/016831.html
Merge pull request #847 from fbudin69500/add-MeshToLabelMap

ENH: Adding new extension MeshToLabelMap