ENH: Add new array joining function: np.interleave

Interleaving arrays is something I need to do every now and then, and by the looks of stackoverflow so do others:

http://stackoverflow.com/questions/12861314/interleave-rows-of-two-numpy-arrays-in-python
http://stackoverflow.com/questions/5347065/interweaving-two-numpy-arrays

I think the code needed for the general `n` dimensional case with `m` number of arrays
is non-trivial enough for it to be useful to provide such a function in numpy, so I took the liberty to
make this PR with my implementation. This would be my first contribution to NumPy so I apologize if something is not adhering to your practices.