[WIP]Adding tests for estimators implementing `partial_fit`

Attempting to fix #3896

New features / tests / enhancements added so far : 
* `method_filter` : ( In `sklearn.utils.testing` )
To filter the estimators based on the existence of one or more methods passed as a list of method names.
The default `method_filter` value is `None`. It can also be an empty list.

Todo:
- [ ] Implement `method_filter` to help separate the estimators which implement any specific method(s) [ eg `partial_fit` ]
( The below taken from @arjoly's comment : )
- [ ] Check that an error is raised if the number of features changes from one partial fit to another
- [ ] Check that doing fit -> partial fit == partial_fit, partial_fit
- [ ] Check that doing fit after a set of fit/partial_fit restart the estimator
- [ ] Check that classifier handles correctly the classes argument in the partial_fit.

@arjoly @jnothman Pl review when you find time... Also edit this and add more tests if you feel that more should be added...