Removed n_jobs parameter from the fit method and added it to constructor

Fixes #3672 
@arjoly I have deprecated the n_jobs parameter from the `fit` method and added it to the constructor. I was not certain as to how to add the deprecation warning though as i am new to this community, could you please tell me how to add that? I will patch that up in the new commit along with the tests if needed Thank you!
yup. we should not modify the class attributes set in the constructor as far as I know till now. that was my intention of saying `self.n_jobs` should not be modified. http://scikit-learn.org/dev/developers/index.html#instantiation . But you can wait for @arjoly to confirm.