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)
In that case you should modify line 1025 to say:
```python
if self._use_dense_features and not isinstance(xtest, np.ndarray):
```