SMTP MIME Email Message decoder

Updated revision 1 - fixed PROTOCOL support in file detectors, separated base64 code into new util, fixed indentation, used existing find function, misc other changes

This module parses email messages and decodes content bodies that are transmitted over the SMTP protocol.
Parsing is done on a line-by-line streaming basis to minimize memory resources.  Some protocol anomalies are
reported as flags in the data structure and file extraction is also available in a streaming fashion using
a callback function.