Re #4311. Return the methods of the proxied object.

This helps tab-completion in IPython return the methods that we want to see.
Re #4311. Return the docstring of the proxied object.

This helps the correct documentation string show up in the IPython console.
I've also turned on automatic docstring generation in our sip call.
Merge branch 'feature/4311_mantidplot_python_completion' into develop