Improve logging

This implements parts of #152.
Mainly the `IPFS_DEBUG` flag and the `ipfs log` command to change log levels of a running daemon. I chose add a explicit `GetEnvBool()` to the `util` pkg.
Was there a bug with how the logic was before (setting `lvl := logging.ERROR`, and changing it if the env var, etc) ? That way `SetAllLoggers` is only called once.