Fixes #14: Add POP_ARTBOUNDARY. Modified MARKARTIFACTS

Added function to detect epochs/trials containing boundary codes and
mark it with an artifact flag.

#14: Modified MARKARTIFACTS to mark the RTFLAG property in
EEG.EVENTLIST.BDF in order to synchronize behavioral RT trials with ERP
trials.
updated MARKARTIFACTS to check for RTITEM field

MARK ARTIFACTS previously threw an error if the ISRT field was true but
no RT fields (e.g. EEG.BDF.RTITEM, EEG.BDF.RTHOMEFLAG) exist. Now if
ISRT is true but no RT fields exist, then MARKARTIFACTS skips updating
response times and does not throw an error.