Added ability to use filters with searchPackage() function.

Hi Javi,

I've been studying package kit filters, and have a version of your packagekitBridge that uses them.  It's backward compatible with what you had before, so you (we [GPII]) can continue to use it as before.  But, now, we can use various filter strings to shape the results if we want.

You can see a limited use of these filter strings in the unit tests.  I plan to look at other filters to see how they might fit in.

This URL gives a quick overview of all of the filters:
http://www.packagekit.org/gtk-doc/introduction-ideas-filters.html