Add Regressor and refactor backend context

[anatolfernandez]

WIP

[mfeurer]

Looks good. I'll merge that soonish.

[mfeurer]

Okay, while merging that I was wondering whether the code would still work with multilabel data. In line 340 you simply use argmax() instead of looping over all outputs. Can you please check that?

While doing this, could you please also rebase the code? Would it also make sense to squash the code into one commit since it is about a single features?

[anatolfernandez]

@mfeurer I'm merging now..

[coveralls]

Coverage increased (+0.3%) to 79.936% when pulling 1805394 on Ayaro:regression into 1c363e6 on automl:development.