fixReport() and interest periods

[davebraze]

Add support for flagging events (fixations) as being either inside or outside of an interest period within a recording block. Will need to identify (via RegEx) events that correspond to beginning and end of interest period.

This will rely on properly gathering MSG events other than trialvars in readELascii(), which is not working at present. See the msgSet argument to that function.

[davebraze]

Progress made in PR #47 by @monicaycli. See also issue #45.

MSG events are now properly read from ELascii file and stored in data.frame. However, parsing is minimal.