So, for now all dependencies of the Framework can be installed
with composer and we need only lib/Roundcube folder.
des.inc was never required and is less important since we do
encryption using openssl or mcrypt extensions.
Rcube_message_part::body content should never be modified by code out of the rcube_message.
Added convenient rcube_message::get_part_body() method, making rcube_message::get_part_content() deprecated.