Fixes #712

While doing this I made also some refactoring to reduce duplication in methods that add attributes to layers and methods that retrieve field names and types.
I also removed QgsExpression.quotedColumnRef where it was not needed. I am still using it in expressions for rule-based classification, to properly quote field names inside the expression, but it is not needed otherwise.
I made changes to a dozen of files, so I have to be careful that everything still works properly.
In some cases I am assuming that fields are numeric, but I still have to double-check that it is true in those cases.