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.
fixed some gpu tests (different rounding results due to float arithmetics)
a few fixes of ocl::perf test cases
Fix ocl::pyrup kernel build on Mac.
Fix ocl::filter2D.
In current implementation, this function only works when anchor point is
in the kernel center and kernel size supported is either 3x3 or 5x5.
Adjust perf_filters, as this function only supports 3x3 kernel
fix a crash on Linux
Update ocl::surf_matcher sample.
The new sample adjust some parameters thus it should always be able to
calculate valid homography matrix when input is box.png and
box_in_scene.png.
Pure cpp surf and bfmatcher implementation is also added to show the user
its accuracy and performance.
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. ;=]
Initialize OpenCL context at the end of getDevice call.
Added for better compatibility with the current samples/test cases.
User now will be able to initialize OpenCL context explicitly with
ocl::getDevice api.
This may be obsoleted in future releases.
Fix build error
Merge pull request #931 from jet47:gpu-test-fixes
Make AutoLock noncopyable (it would break on copying, anyway).
setting 'char' to be signed by default since some tests fail when it's wrong (e.g. native compilation on ARM Linux)
making the comment less ambigous
Merge pull request #934 from SpecLad:parallel-for
Merge pull request #912 from SpecLad:contributing
fixed GPU module compialtion with CMake 2.8.11

CMake 2.8.11 removed linkage with CUDA driver library,
but it's used by gpu video encoding/decoding
fixed NPP library search (it was splitted)