Set a more sane default log level

(for issue #12)

Logging /alone/ takes 50%+ of HTTPS Everywhere's profile time in Chrome (!)

Chrome will log & buffer console.log() even when the console is closed.

This means that for 99.9% of users (who never open the console), Chrome wastes
staggering amounts of time logging and buffering extremely verbose data.

For now, this just sets a default log level and includes adds the .warn() function.

TODO (Upcoming commit): Add a UI element to change the default log level.

Developers can simply type (in the console): DEFAULT_LOG_LEVEL=1;

Signed-off-by: Nick Semenkovich <semenko@alum.mit.edu>