Better-looking histogram checkboxes

This patch changes this:

![current](http://puu.sh/4vJro.png)

to this:

![fixed](http://puu.sh/4vJpX.png)

For whatever reason bootstrap in kibana doesn't have css rule (original bootstrap has) for `.form-inline .checkbox { display: inline-block; }` so I added it. Maybe there's better way to do this.

Interval selector should be fixed too, but I didn't find how to make nice inline select.