Fix most of the errors/warnings in the current docs.

Also sprinkle a bit of pep8.
Add a --cache option to docgen to make it not disable the doctree cache.

This helps immensly when writing or editing documenation by massively
speeding up documentation rebuilds (as in 2-3s versus 60-90s before).
Add a section about GpuArray in the Using GPU section of the tutorial.
Some fixes following comments.
Add numpy "casting" example to the old backend section.
Remove the "Full speed" section and link to the borrow tutorial.

Also add an example of the GPU speedup for borrow to that tutorial.
Finally figured out the header problem.  Now it should be what we want.
Merge pull request #1878 from abergeron/gpuarray_doc

Gpuarray doc