Remove attribute

Remove attribute based on how CarrierWave does it. Fixes #12.

1) Do we want the API do be different from CarrierWave somehow? Should the method still be called `remove_attachment` etc?

2) I took the `remove?` method directly from CarrierWave. I was wondering if the parameter parsing should rather be done by the `define_method "remove_#{name}="`?

3) Should there be a feature test for this?