simpeg/discretize: numpy upgrades, IO for UBC meshes

IO: UBC meshes

contributions from: @thast, @micmitch, review from @lheagy from pr #54

• discretize.TensorMesh.readUBC is now a wrapper for loading either 2D or 3D mesh
• discretize.TensorMesh.readUBC can take care of recognizing if the file is a 2D or 3D UBC mesh, or it can be an input given by the user (arg: meshdim)
• the original functions are now hidden class function under discretize.TensorMesh._readUBC_3DMesh and discretize.TensorMesh._readUBC_2DMesh
• comments in UBC files are assumed to be marked with "!" in front

contributions from: @lheagy, review and comments from @rowanc1, @jcapriot
from pr #61

• Updates to Identity() and Zero() so that they are consistent with numpy upgrades in numpy=1.13.0 (https://github.com/charris/numpy/blob/master/doc/source/reference/arrays.classes.rst)
• use scipy.sparse.dia_matrix for spzeros (closed #60)