Deep copy needed when constructing Widget Model, rather than shallow copy

As per this issue https://github.com/nickholub/angular-ui-dashboard/issues/5

This solution solves the problem for my graphite dashboard and in practice seems to work.

Hopefully my change for shallow vs deep cloning make sense?

The only use case that I know I ignored is what if "overrides" contains objects which need deep copy also...haven't seen the use case for "overrides" yet, so I simply don't know!