A number of changes to how traitlets and components work....
A number of changes to how traitlets and components work.
* Classes can be given as strings, like 'foo.bar.Bar' in the Type
and Instance traitlets as well as the get_instances method of
Component. This is done to allow for forward declarations.
* New IPython.utils.importstring that imports a class from a string,
like 'foo.bar.Bar'.
