Allow filtering based on terms in multiple taxonomies

Hi! I really like this plugin! However on my site I need to filter posts based on terms in multiple taxonomies (a custom taxonomy and tags). I think this feature would be useful to others, so I'm offering my code back to you in case you want to incorporate it into the plugin.

I've replaced the "Taxonomy & Terms" tab with a "Terms" tab that displays all terms in all taxonomies, and allows users to choose whether they want to include posts with all selected terms or any selected terms. 

![fpw terms tab](https://cloud.githubusercontent.com/assets/5298452/4925345/64abf804-652a-11e4-9555-c4eee684463f.png)

There are two issues:
1. I needed to change the way instances' data was stored. This means instances created with previous version of the plugin need to be updated. I've included code to do that, but people will need to re-save their widgets after upgrading in order for their terms to work. If you want to incorporate this change and don't want existing users to have to take that step, I can help with code to properly display old instances that haven't been re-saved and therefore haven't had their data updated.
2. I haven't provided translations for the new strings I've introduced.

I know this is a significant change to the plugin, but I hope you consider incorporating the work, as I believe it will be useful to others.

Thanks for a great plugin, and for writing very clean, readable code!