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. 

please note that there is a significant difference in speed

```python
In [8]: timeit a.update({2: 2})
1000000 loops, best of 3: 482 ns per loop

In [9]: timeit a[2] = 2
10000000 loops, best of 3: 83.1 ns per loop
```