Add r.js' wrapShim to grunt and sbt builds
Ignore useless warnings from closure in sbt build
Move Widget to bokeh.objects and don't import widgetobjects
Pre-load models in Viewable.get_class()

This is required, because model_class_reverse_map is filled when
models are first imported. However, we shouldn't assume that
they will be imported at all prior to first call to get_class().