Rewrite docstrings to match scikit-learn Style

[desilinguist]

We should try and match the way that scikit-learn writes their docstrings. Here is an example.

For example, there is a blank line between the parameters and the default values are indicated right after the parameter type. It's also nice how the types that are numpy arrays also include the shapes.

[desilinguist]

See also this issue for SKLL.