MLContext extensions for trainers

[Zruty0]

Go over all the trainers and ensure that:

• They are named according to Mass rename of transformers and MLContext extensions for them #1318
• Appropriate extensions are present
• The predictor classes are available in non-Runtime namespaces.

I anticipate that there's going to be some naming or interface decisions here, let's discuss them in this issue.

[sfilipi]

Is the Microsoft.Ml.Trainers a good namespace for them, or we want to keep them in something like Microsoft.Ml.Models?

cc @wschin

[sfilipi]

This is complete through #1524, #1682, #1739