Reduce number of false negatives by comparing decoded message bodys

Message bodies encoded as quoted-printable are sometimes reformatted
(probably to conform to a 76 characters per line limit). This does not
change the actual (decoded) content, but confuses the diff algorithm if
the raw bodies are compared. With this patch, the bodies are first decoded
and then compared.
fix non-working -i option
Ignore extra spaces before and after header values

Some duplicate messages I encountered had an extra space before the
message id in one of the copies. Removing leading and trailing
whitespace in all header values fixes this problem.
Merge pull request #6 from marcelm/master

Reduce number of false negatives by comparing decoded message bodys