Additional options for ol3-popup

[Hareck]

Hi Matt,

I have played with your 'ol3-popup'.
I added some additional options like:

• offset // popups offset
• keepInMap // to move the popup into view
• finisher // true/false, popup with or without closer
• backgroundColor // to set up a background for the popup
• borderRadius // popup's borderRadius
• arrow // true/false, popup with or without arrow

They are useful if there different popups necessary.

My result is here: http://www.hareck.lima-city.de/ol3-ls-popup/ol3test.html.

There is one problem, not with the popup, but with ol3: OpenLayers 3 not detecting a feature when over a
transparent pixel of the icon is a feature.
I have opened an issue: Detecting a feature when over a transparent pixel of the icon is a feature (#2961) for discussion.

Thank you for the great layerswitcher and popup implementation.

Hareck