Add friendly message about Python 3.x being unsupported

A bit better than a traceback.  Also introduce environment variable
DEVELOPING_OFFLINEIMAP_PYTHON3_SUPPORT that will allow developers
to grok the code under Python 3.x.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap.py

@@ -17,6 +17,19 @@
 #    along with this program; if not, write to the Free Software
 #    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 
+import os
+import sys
+
+if not 'DEVELOPING_OFFLINEIMAP_PYTHON3_SUPPORT' in os.environ:
+	if sys.version_info[0] > 2:
+		sys.stderr.write("""IIMAPS!
+
+Sorry, OfflineIMAP currently doesn't support Python higher than 2.x.
+We're doing our best to bring in support for 3.x really soon.  You can
+also join us at https://github.com/OfflineIMAP/offlineimap/ and help.
+""")
+		sys.exit(1)
+
 from offlineimap import OfflineImap
 
 oi = OfflineImap()