clogger 2.0.0 - updated website URL, cleanups

This updates the documentation to point to the new homepage
at http://clogger.bogomips.org/

There is one API change which removes :to_io support,
as this was never a valid Rack extension.

This also fixes a minor incompatibility which prevented the GVL from
being released on ruby-trunk (2.2.0dev) during disk operations.

The mailing list continues to be hosted at librelist, but that will
probably soon change to a public-inbox + mailing list.

There are also minor code cleanups.

This also relaxes license to LGPLv2.1 or later.