GH-74 Delay load widgets in marshaller

Currently the marshaller loads the JS for all known widgets upfront - which can
be "heavy" for the Choropleths.
This change only loads the Widgets that are required by the marshaller.

Fixes GH-74

Signed-off-by: Gordon Smith <gordonjsmith@gmail.com>
Merge pull request #81 from GordonSmith/GH-74

GH-74 Delay load widgets in marshaller