Update README.md

adding HipChat link
Remove useless variables
Use the fast channel for the all-agg-columns case
Refactor default functions of aggregation
Move the Python mget() down to Cython
Clean up unused functions
Adding partition_filter
Make partition_filter work with iterable
Fix wait() for Future to be compatible to cat()
Merge remote-tracking branch 'origin/develop' into develop
Bump version to 0.2.3