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

Depending on the use case, Tocsin users may want to rescue *all* possible errors, not just StandardErrors. This change allows that, and also permits future use cases such as only rescuing certain application-defined exceptions.

For example,
<pre>
Tocsin.configure do |c| 
  c.exception_level = Exception
end
</pre>