Feature/explain

Hey there,

I've put together an explain feature for genghis (with tests). This is for issue #60.

Right now the the feature is available as a checkbox in the advanced search area. If you have a better idea for how to enable this in the UI let me know. I think in the advanced area is a good idea because it's out of the way.

I've included the php change too but it's been quite a while since I did any php work so it was pretty hacky.

Any feedback would be great!