Initial work on expression transformation
added the supported demo
Updated the code to utilize the relational map on the property-path remapping.
Refactored UOW with the generic graph sorter
Massive refactoring on how the metadata is stored by consolidating scattered static properties into one metadata object.
updated the documentation
Renamed classes.
Added the incomplete ability to do cross-collection query.

Note that this is a breaking commit. For development, run

make TEST_FLAGS="ft/db/test_driver_mongodriver.py" test_py2

to verify if the query works correctly. Also, this commit
contains the debugging feedback.
Added more property in the join config
Updated the test to fix the broken test
Updated the test to reflect the backward-compatibility breaker
Updated the ORM to use the new Query engine
Removed the debugging code
Updated the setup script
Updated the CI configuration
Merge pull request #39 from shiroyuki/dev-expr

New Query Engine, Cross-collection Query, Driver-driven ORM