Fix bug with warm_start in CDRegressor.

Fixes #9.
Basic implementation of PRank.
Add Cython implementation of PRank.
Add shuffle option.
Add KernelPRank.
Add docstring.
Fix typos.
Add LinearRidge and KernelRidge to documentation.
Add ranking section to doc.
FIX: Raise error for non-elastic net penalties and when multiclass is True
Merge pull request #11 from MechCoder/exception_multiclass

FIX: Raise error for non-elastic net penalties and when multiclass is True