Add prechew data functions
Create derived slices!

Seriously, this is awesome. In your dataset definition, you can define
new slices by aggregating other slices. In fact, I added a dependency
graph so that you can derive slices from derived slices and it'll all
work out.

TODO: Allow WHERE clauses
Allow for filtering when creating derived slices
Added name and description of slices to API pages
Merge from master
Fixed broken integration tests
Merge pull request #30 from cndreisbach/aggregated-slices

Aggregated slices