Add support for tags as list of strings


Problem description
===================

Sometimes the Angular model tags are stored as list of strings.

This add an ui-select2 specific option `simple_tags` which force keeping the model as a list of strings.

This implements a very specialized data binder as discussed here https://github.com/angular-ui/ui-select2/pull/7


Changes description
===================

To inform ui-select2 to keep data as list of strings, the `simple_tags` option was added.

Documentation was updated with a short introduction of `simple_tags` options.

2 tests were disabled, since I am not sure if they really need to use the 'val' call.
If required I can update the tests.

Tests were added for simple_tags usage.


How to try and test the changes
===============================

Check the example from documentation.

Please let me know if you think that this is the right thing to do, or it is better to implement this using generic data hooks as discussed here https://github.com/angular-ui/ui-select2/pull/7

Thanks!