Fix an old two-arg let that slipped by.
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.
#88: parse BETWEEN as a function of three args, and handle it in the planner
delete unused extractor
Merge pull request #90 from mossprescott/three-arg-between

#88: Three arg between
Merge branch 'master' of github.com:slamdata/slamengine into between