COUCHDB-2158 Fauxton: Implement a filter for changes

Implement a filter for changes plus some cleanup afterwards

@garrensmith @deathbearbrown what do you think of my second commit where I try to reuse the duplicated filter code? Maybe there is a better solution, I added it to the view-constructor-function, but maybe I should create a special `FauxtionApi.FilterView` or something like this, what do you think?