Add access to cuda context handle

Update  compiler.py

Solves error generated by using sys.maxint constant in Python 3.1.5^, sys.maxsize is available for use in Python 2.0.0.

Python docs for reference:
- sys.maxint deprecated in Python 3.1.5^: https://docs.python.org/3.1/whatsnew/3.0.html#integers
- sys.maxsize supported in Python 2.6^ : https://docs.python.org/2.6/library/sys.html#sys.maxsize

Ensure the views from DeviceAllocation.as_buffer are writeable.

Surf an dtex extensions

Adding complements to use Texture and Surfaces 2D Layered

Allow the caching behaviour to be controlled globally.

The kernel compilation cache can be disabled by exporting
$PYCUDA_DISABLE_CACHE and the default cache directory can be
overridden by exporting $PYCUDA_CACHE_DIR.

Zero-initialize all fields of Memcpy*D

Some fields were left uninitialized, which doesn't seem very Pythonic.
The CUDA_MEMCPY3D structure is now value-initialized, which ensures all
its members are zero.

Add ability to get handle to CUDA Arrays