Merge pull request #1 from ddemidov/master

A couple of improvements in VexCL code
major modifications. documentation to be updated
Treat slice in the same way as the rest of members
Initialize members in the order of declaration
No need to keep kkrev, since it may be computed on the fly

Global memory IO is much more expensive than arithmetic operations.
Optimize Chebyshev construction

Again, there is no need to store intermediate values to global memory
when those could be computed on the fly.

Also replaced boost::math::constants::pi with vex::constants::pi