support ng-required validator - fixes #258

[insidewhy]

fixes #258, at least for single-entry ui-selects.

I took the work from @zgbjgg but applied to the correct files in src and fixed the whitespace/grammar.

I've verified the tests still pass with gulp but didn't include the changed files in dist in that commit. Would be very grateful if you could release 0.19.2 with this fix.

[insidewhy]

Let me know if you want me to commit the files in dist and if so, whether I should merge the commits together or apply the updated dist files in a second commit.

[insidewhy]

Sorry ignore this PR for now, after testing the changes they don't seem to work for me.

[insidewhy]

I'll try again with a new PR based on my own work... I'm not seeing any of the form[fieldName] fields.

[user378230]

You may want to review angular documentation for ngRequired: https://docs.angularjs.org/api/ng/directive/ngRequired

Note all PRs need tests as well.