[doc] Mention submodules in the build instructions
[ots] Add a compatibility wrapper for old API

To help with Chrome transition, should be removed after that.
[ots] Distinguish between errors and warning

Add a level parameter to the message callback that allows the clients to
handle errors and warning differently.

Related to #36
[ots] Disable bogus changes to metrics tables

Together with the warning messages.

Closes #36