We should be able to clean a single field or a subset of fields.
This will be required to implement #6 if we want to avoid having to re-clean the entire form.
Implemented partial cleaning for #29
Moved the guts of _cleanFields into _cleanField for cleaning of individual fields
updateData() updated to handle forms with prefixes set
Partial cleaning is now implemented as updateData() and partialClean - turns out it's only part of the story for #29, but still...