mongo: add into, discover
mongo: into mongo <- data descriptor
fix mongo tests
api: Table doesn't check schemas if .schema isn't a datashape
mongo: add basic compute system
mongo: better selection/projection tests
mongo: refactor match, head test
mongo: simple by operation
mongo: reductions
mongo: sort
mongo: add to imports
mongo: into Collection <- TableExpr
add mongo to travis script
travis: mongo - sudo apt-get mongodb-server
travis: oh, mongodb is on conda
travis: use travis mongodb
mongo: py3 compatibility
py26 compatibility
mongo: docstrings
py3 compatibility