Added numpy three_corner_hat method
Moved numpy methods to new file
First full numpy-ified version of allantools.py

Finally figured out a numpy-ish way of computing MTIE. Unlike other
methods, the speedup is only modest (3x). Getting this to be faster
will require a quicker way to do the rolling window.

The new code passes the runtests.py test routines. The old pre-numpy
code has been moved to allantools_pure_python.py; I think this code
should stay as it's a good reference and shows the different ways
of thinking: loops vs. vectors. I've certainly learnt a lot by porting
it over to numpy!

Next on the agenda will be adding numpydoc docstrings for all the
methods, and then generating sphinx documentation. Then this will truly
be the most awesome toolkit out there for measuring time stability :)