Convert helper methods for AbstractUploadable
Add virtual PreProcess method

Make changes to the rows before import or no-op
Pre/Post process project values for filter lists
Cache attributes call

Add more aggressive caching to attribute/value requests.  Invalidate
cache
when new data is uploaded.
Merge pull request #41 from azavea/topic/attribute-caching

Cache attributes call
Merge pull request #40 from azavea/topic/project-post

Pre/Post process project values for filter lists