SelectView should work properly with optionValuePath and selection

SelectView was not working property when `optionValuePath` and `selection` properties were set. 
`selection` and `value` properties did not update, also `selected` attribute was not set on option tag. 

Typical use case in which SelectView content was bound to ember-data records array and selection was bound to some ember-data object fail to set Select into proper state. Example: http://emberjs.jsbin.com/zemomorunu/1/

This PR aims to improve SelectView so its content can be bound to array of any Ember objects.

TODO:
* support for `multiple=true`
`content` may be intentionaly set to null, so `getWithDefault` wont help here