Remove DIMP_ prefix
End sandbox eperiment

Problem is that pages opened from the IFRAME sandbox also have limited
behavior (e.g. javascript is broken) so this is not a viable solution
DimpCore -> ImpCore
DimpBase -> ImpBase
Remove dimp prefix from CSS classes/DOM IDs
Missing since information for Horde_Smtp_Filter_Body
[mms] Added Horde_Mime_Filter_Encoding class.
This fits better in Horde_Mime
Another UPGRADING blurb
Better encoding defaults