Add placeholder to multi-select and fix the tiny 25px selection area.

@azirbel would you please help me review this? 
Basically, I'd like to fix the problem of having a small area we can click into in multi-select field. One easy fix is separate them, and make the input field occupy 100% width as I do here.
We may also want to put them (the search box, and the current items list) in different color for guiding users to click into the search box. But I'm not sure if we should put the style into ember-widgets. I overwrite it in iverson
Here is the corresponding commit from iverson:
https://github.com/Addepar/Iverson/commit/a6e0886989cbb22f28f902c0f5c78cf9aaef2a23

Thanks.