More efficient code for lines for trial events

Previously had separate code for each line corresponding to trial
events. Needed to refactor code to accommodate for two plots, each
corresponding to the rule. Noticed problem with spikes not being colored
correctly (or removed correctly?).
More efficient code for labels for trial events

Updated code for trial event labels. Also renamed some things to be more
readable.
Changed label positioning to mean event times
Added functions for appending axis and label

Also removed commented out code and increased the distance between plots
(plot_buffer).
Remove prior axes on redraw

Remove prior axes before placing new ones.
Rewrote spike plotting code to use .each()

Previous method of code was ugly. Using d3.each makes the code much
cleaner. Moved all of the spike plotting code of vis.draw into a
subfunction called by .each().
Moved colorScale settings into a subfunction
Added plot names