Merge pull request #84 from sellout/13-mongodb-equi-joins

Fix an old two-arg let that slipped by.
Update the README.

Bring in line with the current behavior, reorganize a bit, and add some content.
Merge pull request #86 from sellout/minor-fixes

Update the README.
First swipe at MapReduce evaluation.
Add support for PureTask to insert a set of documents.
Add FoldLeftTask.

For sequential composition of tasks.
[minor] Formatting.