add filterNames

[mulderp]

A small wrapper to retrieve the names of the filters

[jmorrell]

A couple of points:

• This functionality absolutely makes sense to add, however
• The names here, don't really fit in with the existing convention [*] so I'd likely change them to getFilters and hasFilter
• You've modified the backbone.obscura.js file directly, which is actually not what you want since it's a built file. We need to first modify backbone-filtered-collection, the smaller library that obscura builds on top of. I'll see if I can make this setup clearer in the docs.
• Thanks for including updated docs + tests in your pull request!

I'll try to merge this in and get things sorted out later today.

[*] By that same logic, I just realized that filterBy should probably also be changed to setFilter

[mulderp]

Interesting! Hmm.. maybe "addFilter" would be nice too, since we have "removeFilter()". "setFilter" sounds more like a setter of a property, but since we can have a list of filters, add/remove somewhat clarifies that.

I also can update this PR later today or in the next days.