Adds a CompressableField abstract base class from which fieldtypes may inherit. Fields of these types may specify compressed="true" in the field definition to turn on compression. The field type takes an optional compressThreshold parameter which will only turn on compression for fields whose internal representations of greater or equal to a given size.
TextField and StrField inherit from CompressableField
Comments? (my main issue is that it woudl be nice to be able to set compressThreshold at the field level. Since we can't without modifying fields to accept named parameters, perhaps it would be preferrable to only allow compress=true/false to be set at the fieldtype leve)