Minor tweak to be more idiomatic
Move encoding/filter string generation

Both of these functions are really static
Xcode doing its thing...
Create filter objects from OTM2 instance info endpoint
Remove missing fields hardcoded stuff

OTM2 has a much more simple way of identifying "missing" fields. So we
don't need all that rigmarole that we had before
Merge pull request #55 from azavea/topic/filters

Topic/filters