Add .iconpicker-popover class

Add `.iconpicker-popover` to the created Bootstrap popover to enable custom CSS overrides. I have implemented probably the most common use case (which was my original requirement) - to increase the z-index to display in front of Bootstrap modals. 

My editor has auto-removed a lot of extra whitespace and added trailing newlines. I am happy to re-submit without those automatic changes if you prefer.

Minification was done with [minifier](https://github.com/fizker/minifier), all default options.

The actual changes are JS line 827, CSS line 30.