Clean up logging levels

[glennmatthews]

Python standard seems to be to have three 'levels' of verbosity:

--quiet: WARN/ERROR
(default): INFO/WARN/ERROR
--verbose: DEBUG/INFO/WARN/ERROR

COT's current behavior:
(default): ERROR
-v: WARN/ERROR
-vv: INFO/WARN/ERROR
-vvv: DEBUG/INFO/WARN/ERROR

We should follow the standard instead.

Additionally, the levels are not always used consistently - in particular the DEBUG messages are extremely verbose while the INFO messages are often lacking.

[glennmatthews]

Could be nice to have a "verbose" level between "INFO" and "DEBUG", as in https://pypi.python.org/pypi/verboselogs

Wonder if we can also benefit from https://pypi.python.org/pypi/coloredlogs/ by the same author.