"popoverTemplate" and "popoverTemplatePopup" directives

Addresses feature request "support template url for partial" https://github.com/angular-ui/bootstrap/issues/220

Improved over @joshdmiller PR #369

Resolves properly the ng-model binding issue seen at http://plnkr.co/edit/cbqOnktHhxSjeLIBE1w7?p=preview