fix(datepicker): using is-open attribute instead of open

BREAKING CHANGE: 'is-open' replace 'open' attribute for the datepicker popup directive.
  Open is an HTML5 reserved attribute which angularjs 1.1.x protects.

  To migrate your code, replace simply replace open by is-open in datepicker-popup directives.

  Before:

  <input ng-model="date" datepicker-popup open="open" ...>

  After:

  <input ng-model="date" datepicker-popup is-open="open" ...>