Processing_GUI: more improvements for serial comms

More tweaks to the code to improve serial comms.  Also, redefined when
the GUI decides to redraw the screen.  This helps to smooth the display
a bit, though it's not perfect.  Code now interprets and writes the
sample counter provided by OpenBCI/Arduino.
ADS1299Manager: now it streams the sample number

In each data packet, it now includes the sample index.  You can now
watch this index to see if there are any hiccups on the PC in the
reception of the data from the OpenBCI/Arduino.