Implement multipath queries.

This PR implements multipath query for collections.

Fixes https://dev.plone.org/ticket/13251

I know that @tisto implemented something similar in https://github.com/plone/plone.app.querystring/pull/10.

But imho it's better to have multiple path criterions (multiple input fields), than entering a list in one single input field. This is more readable for the user and we don't have to change/improve the widget. 


Before:
Searches only in the second path `/news`. Means only one result.

![screen shot 2014-01-29 at 11 17 54 am](https://f.cloud.github.com/assets/437933/2028442/a8985118-88ce-11e3-9475-7abcfcd51ec1.png)


After: 
Multipath query for `/events` and `/news`. Shows both items.

![screen shot 2014-01-29 at 11 13 14 am](https://f.cloud.github.com/assets/437933/2028413/0b772a8a-88ce-11e3-9a84-75754ee37602.png)