Small fixes and enhancements from dozeo

Hey there,

we've been using CouchPotato for 2 years now and have made a few small fixes and enhancements in our fork.

### Fixes
- added `sum` function to the `MapReduceToMatcher` so unit tests that use it in the reduce function work correctly (616e2f7)

### Enhancements
- made database host configurable (ae52f2a)
- `use` method for CouchPotato that can be used to work with multiple databases in one project (eaadf37)
- fetching models in views that return instances of different classes (but have the same superclass) work correctly (f83da73)

I can also split these into separate PRs but I'd like you to take a look first. :)

Thanks for an awesome project