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 change this `ValueError` to given an error message like:

```python
raise ValueError('An invalid sample was specified: {}. '.format(sampler) +
                 'Valid samplers are: {}'.format(', '.join(_VALID_SAMPLERS)))
```