Import central classes at higher levels in the package hierarchy

Very small change to expose some of the central classes at a higher level. The aim here is to decrease the verbosity of the average nipype script, at least a little bit. In other words, instead of 

    from nipype.pipeline.engine import Node, Workflow
    from nipype.interfaces.io import DataGrabber, DataSink
    from nipype.interfaces.utility import Function

One can do

    from nipype import Node, Workflow, DataGrabber, DataSink, Function

or

    from nipype.pipeline import Workflow
    from nipype.interfaces import Rename, Merge

I can't see how this would cause any problems (tests look fine), but someone should double check.