Add AMOS sources and convenience wrappers

This addresses #10.

The AMOS special function library (http://netlib.org/amos/) was added. Some modern wrappers and tests for those are added as well. AMOS offers the Bessel functions J, Y, H, I, K for real orders and complex arguments. Also, it allows computation of the Airy functions Ai and Bi as well as their derivatives.

The code is tested against SciPy's output with something close to machine precision. SciPy wraps around AMOS as well. For convenience of the user, negative orders are supported by the convenience wrapper.

If it helps reviewing I am happy to squash all commits onto only one.