Fixed handling of Long values, removed support for Floats

MongoTokens had a bug in handling of Long values (and Floats), since it
did not correctly encode them as "second class primitives" (objects
used to represent primitive types that have no equivalent in mongo).
However it turns out that Long values CAN be unambiguously encoded in
mongo, as "NumberLong". This allows us to handle them without the
concept of "second class primitives". Support for Floats has been
dropped for now, since it makes more sens to use Doubles instead, for
Nodes that will be persisted to mongo.
Updated favicon readme

... to reflect that no browsers seem to support SVG favicons.
Added partial support for extended sessions

System is loosely based on the Lift extended sessions for mapper.
Currently works with the exception of an annoying bug that when
automatic login is performed, the cookie deletion and recreation does
not seem to make it to the browser, leaving it with an old cookie. This
could be worked around by not renewing the cookie on every automatic
login, but this would be less secure. Note that the same cookie system
seems to work fine for Lift's built-in extended sessions, and S.notice
calls at the same time as the S cookie calls succeed.