Fixes to work with setup.py.

- Fixed a typo in the graphite module in a function name.
Make the readme slightly easier to read.
Make tags based on storage-schemas.conf change from : to _.
Fix the setup.py, which was templated for a different project
Add support for only reading tags, added in kairosdb 0.9.2 or so.
Saves on traffic for getting retention info.
On Cassandra kairosdb only allows [A-Za-z0-9.-/_].

Change the tag for retentions to turn ":" into "_" and handle that
when reading and writing metrics.