Minor bug fixes and updates. Line number tracking.

- Line number tracking was getting truncated at 255, since it was using
wrong variable type. Fixed it with a trunc().

- Increased the max number line allowed by Grbl to 9999999 from the
g-code standard 99999. The latter seems to be an arbitrary number, so
we are allowing larger ones for at least one known use case scenario.

- Created a new test directory to contain some testing g-code to proof
the firmware. Only got started with one test case so far. More will be
inserted as needed.

- Some other commenting updates to clarify certain aspects of the code.

test/test.py

@@ -0,0 +1,25 @@
+import random
+import serial
+import time
+ser = serial.Serial('/dev/tty.usbmodem24111', 115200, timeout=0.001)
+time.sleep(1)
+outstanding = 0
+data = ''
+while True:
+    time.sleep(0.1)
+    data += ser.read()
+    pos = data.find('\n')
+    if pos == -1:
+        line = ''
+    else:
+        line = data[0:pos + 1]
+        data = data[pos + 1:]
+    if line == '' and outstanding < 3:
+        while outstanding < 3:
+            ser.write("G0 Z%0.3f\n" % (0.01 * (random.random() - 0.5)))
+            #ser.write("M3\n")
+            outstanding += 1
+        continue
+    if line == 'ok\r\n':
+        outstanding -= 1
+    print outstanding, repr(line.rstrip())