A few big changes in this commit: 1. The current indexer has been moved to backends/indexer_dbm in anticipation of my writing an indexer_rdbms, 2. Changed indexer invocation during create / set to follow the pattern set by the metakit backend, which was much cleaner, and 3. The "content" property of FileClass is now mutable in all but the metakit backend. Metakit needs to be changed to support the editing of "content". Hey, and I learnt today that the metakit backend implements its own indexer. How about that... :)