sampler addded with tests cases and documentation.

Added support for kernels: 
 * Radial Basis Function Kernel (RBFSampler)
 * Additive Chi Squared Kernel (AdditiveChi2Sampler)
 * Skewed Chi Squared Kernel (SkewedChi2Sampler)
Please make this list of samplers a `frozenset` much like `_VALID_TASKS`, and just check `if sample not in _VALID_SAMPLERS:`. That way the list won't get created repeatedly and checking for membership of a set is much faster than a list.