Include creation date in model metadata

Introduce getter for new object "date" in ModelMetadataJSO, which
captures the return from the "models/open" API call.

Also include new test in ModelMetadataJSOTest.
Display alert when model successfully deleted
Don't allow model to be saved with an existing name

An alert is shown, asking the user to use a different name for the
model.

This fixes #81 in csdms/wmt.
Build Open droplist with populateMenu method

Instead of mechanically building the menu from all the available model
names. This sets the stage for subsetting on models that are owned by
the current user.
Don't allow duplicate model name on "Save As"

This is a follow-on to commit fcd082711ef2272a7a153b889321f4ce0944dcca.
Refactor ModelActionPanelSaveHandler#onClick

Update comments and try to make the code clearer.