Fixed datepicker.js to use data-date-* attributes correctly

Angular-strap was not using any custom attributes (such as data-date-start-date).

The array passed to angular.js was using the correct names for the attributes of the options[] array, but those were incorrect keys for the attrs[] array.  Each attribute of the attrs array has 'date' prefixed to the key.  I couldn't just do attrs['date' + key] because the next character is changed to uppercase in attrs[].

This is a simple fix to make it work without parsing anything.