[First #473] Moved references of explore and made callable from declarativewidgets.explore

[poplav]

This should be rebased after https://github.com/jupyter-incubator/declarativewidgets/pull/473

Fixes https://github.com/jupyter-incubator/declarativewidgets/issues/467

• Renamed widget_visualizations to explore and moved to util dir
• In Scala/Python simply do import declarativewidgets.explore/from declarativewidgets import explore
• In R the explore function is exported to the global environment already

[lbustelo]

LGTM