Rename methods and alias old ones

Some better method names. This PR renames

`Model#classes` to `Model#classes_count`
`Model#support_vectors` to `Model#support_vectors_count`

The problem with the current names is that they seem to imply to return collections of object where in reality they only return counts.

Old names are aliased to the new method in order to keep the interface from breaking.

Any thoughts?