Added option for using FFTW in examples

closes #18
Still not checking if FFTW is found, but its disabled by default.
Replaced cl::__local with cl::Local

refs #19
These two come from CL/cl.hpp and are identical. We can safely use newer
version, since we provide CL/cl.hpp with VexCL.
Got rid of enqueueMarker

refs #19
This should deal with deprecation warnings.
Merge branch 'master' of https://github.com/ddemidov/vexcl
Split index and seed into two arguments.