Fixed bug introduced when I started checking whether the printed
upload date was naive. display crashed when last_upload_date was
none. Now properly prints None when last_upload_date is None.
Bugfixes and improvements as I reimplemented fogbugz_2_fb in terms of
the library.

Default FogbugzInterval representation prints (not deleted) rather
than (present) as it seems a clearer representation of what the
underlying code is saying.

Removed notes on the error messages now that the error messages are
printed in the exeption handling code in fogbugz_2_fb.py

Added missing "self" param to list_intervals_since and removed
incorrect comment line.
Reimplemented fogbugz_2_fb.py in terms of foglib