Replace the pull request guidelines with a link to the wiki.

I've moved the contents of CONTRIBUTING.md to the wiki (and slightly
expanded it), so the former is no longer required. I've put a link to the
wiki page and a summary in the README.
Replaced most of the usages of parallel_for with that of parallel_for_.

This should allow many algorithms to take advantage of more parallelization
technologies.
Split CLAHE into its own file, because it's faster that way.

Yes, it's as ludicrous as it sounds, but it's still true. Bizarrely,
the previous commit makes CLAHE run about 10% slower on Android, even
though it doesn't even touch any CLAHE code. Splitting it off fixes that,
although the reason it does is a mystery for the ages.

It's cleaner when it's in its own file, anyway. ;=]
Merge pull request #934 from SpecLad:parallel-for
Merge pull request #912 from SpecLad:contributing