ngSelectOptions should accept expressions to allow passing any object as select value

Solves https://github.com/pc035860/ngSelect/issues/3

Generally instead of having strings as option values we evaluate expression on current scope and store result as option value. It feels much more Angular-way for me and provides much more flexibility. 

ATTENTION: This PR breaks backward compatibility! You should update all ngSelectOption's to be an expression instead of interpolation. E.g. `ng-select-option = "{{ mymodel.myvalue }}"` should become simple `ng-select-option = "mymodel.myvalue"`. 

TODO: This PR leaves last 2 demo examples from README not updated since I can't test it without new version in public access. Those plunks should be updated as described in paragraph above.