Make vmtkmeshmerge easy to extend to any number of meshes.

Currently set to up to 7 meshes, only need
to change a single number in the file
../../vmtkScripts/contrib/vmtkmeshmerge.py
to extend to more.
Merge pull request #67 from martinal/martinal/mergemultiplemeshes

Make vmtkmeshmerge easy to extend to any number of meshes.