CUDA framework for menpo

The aim of this framework is to simplify as much as possible the development and use of future modules written in CUDA. The idea was to consider that the choice of using or not CUDA should be made Python-side in order to allow the user to choose between the classical algorithm or its CUDA implementation.

Checking if CUDA is available or not is quite straightforward:
```Python
from menpo.cuda.tools import is_cuda_available
if is_cuda_available():
    #run CUDA version
else:
    #run non-CUDA version
```

Adding CUDA modules is also intuitive. It can be done by adding the `*.pyx` file in the list `cython_cumodules` of `setup.py`.
* `from Cython.Build import cythonize` will be moved to `if not on_rtd`
* `import glob` should be replaced by `from glob import glob` and moved to `if not on_rtd`
* `from os.path import join`, `from distutils.extension import Extension` and `from cuda_build_ext import locate_cuda` should be moved to `if not on_rtd`
* `from setuptools import setup` and `import subprocess` have to be removed as duplicates or unnecessary statements