Use a dictionary-style interface for setting, adding and getting of attributes of the mesh

[aaarne]

This pull request adds a dictionary-style interface to the mesh class.

Before this PR

mesh = pymesh.form_mesh(...)
mesh.add_attribute("attr1")
mesh.set_attribute("attr1", compute_the_first_attribute(mesh))
attr = mesh.get_attribute('attr1')

After this PR is merged

mesh = py.mesh.form_mesh(...)
mesh['attr1'] = compute_the_first_attribute(mesh)
attr = mesh['attr1']

Question:

Is it maybe better to return the correct shape of the matrix returned by __getitem__? It might be more straightforward to use then for the user. In order to implement this feature it is required to keep track of the attributes being vertex, edge, face, voxel attributes, which is not yet there in the class. What do you think, would that make sense?
Thank you :)

[ekalosak]

LGTM but needs unit test