Implement alternate DB backend

[fsteeg]

Work in progress based on MongoDB and Casbah (see branch 'casbah')

For portal support, this requires Lucene/Solr integration

[fsteeg]

Wrapping up work on this for now. Experiments are available in the casbah branch: https://github.com/spinfo/drc/tree/casbah

Storing and loading user and page objects basically works (see SpecCasbah). Benchmarks with a single volume show that Casbah needs about 1/3 (w/o images) to 1/6 (w/ images) of the time Exist needs. Editor integration would require substantial changes in SearchView (it currently gets the XML ids from Exist first, then the actual objects etc).

Other areas that woud need updates for a full DB move include the user DB, metadata storage, and portal code.