Remove stray underscore from create_from_gl_texture

Reworked how PyOpenCL module finds its path

Updated the include path generation function to use Distutils first and
fallback to parsing the __file__ string, if necessary

Previously, `pkg_resources` was used to find the location of the PyOpenCL module so that the include path could be constructed.
This commit replaces that by simply using the __file__ and os.path utilities

This change fixes an incompatibility with PyOpenCL and pyinstaller/cxfreeze/etc. When frozen, distutils is not able to resolve the PyOpenCL package directly and subsequently raises a DistributionNotFound error. Since we are the PyOpenCL package, we don't need to be using distutils to resolve it. This issue was [reported](https://lists.tiker.net/pipermail/pyopencl/2014-October/001832.html) by someone else in the pyopencl mailing list as well.

fix crash when using dtype=float16 (assigned tuple instead of integer).

Fix indexing with reverse slices

An earlier commit fixed a rounding issue for slices with steps > 1, but it broke reverse indexing, where step < 0.  This fix should re-enable reverse indexing.

Fixes to enqueue_copy_buffer_rect

There are two bugs:
1. If src_pitches or dst_pitches is specified, there is an uninitialised
   variable.
2. If ConstBuffer is given a short array and content==0, the internal
   buffer is not zero-filled, causing garbage to be passed on.

The latter probably affects a lot of other functions, but hopefully in a
positive way.

Fixes #105.