Make it harder to abuse exitmap.

By default, sleep periods between circuit creations should be high enough to
not bother web services.  Otherwise, exitmap could accidentally be turned into
a DoS tool.
License exitmap under the GPLv3.

Since exitmap incorporates code from MaxMind and SocksiPy which is released
under the BSD 3-clause license, we relicense that code under the GPLv3.