Added option to keep search results after select close

This PR extends [PR#1273](https://github.com/ivaynberg/select2/pull/1273) and addresses issues [#694](https://github.com/ivaynberg/select2/issues/694) and [#1347](https://github.com/ivaynberg/select2/issues/1347)
Also, usage of this option fixes incorrect scrollbar position after re-opening select.
One thing I can't understand is a line:
```
this.search.val(this.focusser.val());
```
I've commented this line in my PR because I do not understand it's purpose.
Comments are welcome.

PS: git shows many changed lines because it takes into account whitespaces in diff. Use 'git diff -w' to view actual diff.