Feature mesh

Dear all,
Here is the implementation of the stand-alone mesh.
Files:
 - abstractmesh.py: abstract interface for the class
 - mesh.py: current implementation of the mesh and the different types of elements.
 - test_mesh.py: the set of unitary tests for the class.

Please be wellcome to review the code and point out any suggestion or improvement.
Br. 

yeah, that looks right based on what was discussed (#19). 

However, I think `uuid.uuid4()` instead of `uuid.uuid1` is what should be returned but it would be better if others (@itziakos, etc) give input on this.