Allows Tocsin to be configured to rescue a varying scope of errors.

Specifically, Tocsin.configure { |c| c.exception_level = Exception } will cause Tocsin to rescue *all* exceptions, even things like OutOfMemoryError or LoadError. Use this level with caution.
Moves default config values into Config; refactors slightly to set defaults in the initializer.
Select logger in initialize rather than accessor.
Merge pull request #1 from tocsin/rescue_exceptions

Allows Tocsin to be configured to rescue a varying scope of errors.