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)
Currently our backward-compatibility unit test is failing because `self.sampler` doesn't get initialized to `None` in `Learner.from_file` when it is missing from the loaded instance.

In `Learner.from_file`, please change

```python
elif skll_version[0] == VERSION[0]:
    return learner
```

to

```python
elif skll_version[0] == VERSION[0]:
    if not hasattr(learner, 'sampler'):
        learner.sampler = None
    return learner
```