[1093] HABTM Select Filters

For #1093

Besides fixing dropdowns for HABTM, these changes make it so `id` isn't necessary in your filter. Assuming Foo HABTM Bars, the below now works as you'd expect:
```ruby
ActiveAdmin.register Foo do
  filter :bars
end
```

#### TODO
+ get multiselect working
+ force Formtastic to get rid of the mysterious hidden input that it's
adding