Use info as default logging level

[dpordomingo]

required by https://github.com/src-d/sourced-ce/issues/142

When approved, the logging level of gitbase, gitcollector and ghsync will be configurable with LOG_LEVEL environment var, e.g:

$ LOG_LEVEL=debug sourced init

will make that those components log all messages, and

$ LOG_LEVEL=error sourced init

will make that those components log only error more severe messages.

The rest of the components are blocked by certain issues, as described in causing https://github.com/src-d/sourced-ce/issues/142