is_cuda_available() moved to exposed part of the API

As pointed out by @jabooth on the PR#383, the file hierarchy should follow this pattern:
* menpo/cuda/__init__.py, purely used to expose or hide API. Very rarely contains code.
* menpo/cuda/base.py: contains high level interfaces for `CUDA`, shared functions..

Sample code becomes:
from menpo.cuda import is_cuda_available
if is_cuda_available():
    #run CUDA version
else:
    #run non-CUDA version
cythonize called and imported only if necessary
Remove and move Python imports
Apply 'PEP8 - 79 character max line rule' to menpo.cuda
Fix menpo.cuda documentation
Typo in a comment
Maximum Line Length rule, from PEP-8, applied to setup.py