popover-template directive - addresses issue #220

Addresses feature request "support template url for partial" #220

Improved over @joshdmiller PR #369

Updated from @jbruni PR #1046 for version 0.7.0

Resolves properly the ng-model binding issue seen at http://plnkr.co/edit/cbqOnktHhxSjeLIBE1w7?p=preview