Review: Add choices grouping using 'group by' expression

[dimirc]

WIP

This PR is based on #49 but with some changes based on the comment I left there.

Basically navigation with up/down key is fixed in "grouped mode"

TODO:

• Some tests are failing
• Make a demo with iterator function
• Choices template needs to show correctly only simple/grouped block
• Check correct format of all themes
• Update README with demo of this feature

Demo plunker

cc: @just-boris

[dimirc]

@just-boris I rebase with last changes of your #49 and did some minor changes, basically:

• Add a new demo-groupby.html to show demo of different grouping options. Demo plunker
• Tweak _ensureHighlightVisible() to make group header visible when going all the way up with arrow key
• Separate the way simple/grouped element is shown at choices template. (since we couldn't transparently use ngIf at template and be able to get the element at linkFn, I did some element manipulation using their class names)

There are some new failing tests that should be checked. I guess they appear because of the changes I did at the choices template

[just-boris]

@dimirc you have done a good job!
I've tried to fix a tests, you are right, changes in templates make a deal. Should I update my pull-request with recent fixes, or you do the rest of work yourself?

[dimirc]

The changes at the templates to separate simple/grouped blocks will involve too much effort and additional code that might not be worth (select2 theme was working ok, but the 2 other themes needed some additional fixes)

We'll continue with the approach from #49 and with some extra changes at #102