define a clear set of APIs

[ocean1]

We want to have a clear structure, which will require minimal tinkering with the internals of crave for simple analyses, but also make it easy to create more complex set of scripts.
We need to evaluate switching DB backend from a key-valued store --which we might still support for a limited set of operations-- to something else, document the db structure, and allow low level operations (this is useful when we scrape tons of data, and want to run part of the analysis as an aggregation pipeline, or map reduce operation).