Fix trait_documenter for properties

[mdickinson]

This PR fixes the trait_documenter Sphinx extension so that it no longer fails on regular Python properties. It does this by modifying the can_document_member function so that it doesn't recognise generic traits. This makes sense, since a generic trait is deliberately designed to behave like a standard Python object attribute.

Mostly fixes #1238, but independently of this fix, it would be good to fix the trait-documenter to be less fragile, so that if it fails in some other situation in the future then that failure doesn't prevent the Sphinx build from completing.

EDIT: See #1247 for the "make trait-documenter less fragile" fix.

[rahulporuri]

LGTM

[rahulporuri]

Two comments for the record that are implicit but will be good to document explicitly :

• Ignoring the python properties in the TraitDocumenter doesn't mean that they don't get documented at all. They simply get passed on to other sphinx extensions e.g. apidoc which document them.
• Ignoring all generic_trait types in a class name as a proxy for python properties can be considered overkill but in practice, there are few, if no, examples where users explicitly use generic_trait. So this assumption makes sense for now - until someone who heavily uses generic_trait complains.

[mdickinson]

Thanks, @rahulporuri. I'll merge this into master, then resolve the conflicts with #1247.