Merge pull request #1 from ftartaggia/master

generic method call bug fix and stylistic adjustments
Fix method name
Fix references to constants
Fix reference to _*_screen_init methods
Update measurement screen labels
Add custom characters to LCD controller

See also:

 * http://www.quinapalus.com/hd44780udg.html
 * http://wiki.micropython.org/contributedmodules/charlcdplate
Add waiting message between transitions

If the transition occurs while a screen thread is working there's a waiting
time before this thread ends, the message "Please wait..." gives feedback
about this transition time.
Add screen classes (draft)
Rename ammonia-rpi.py to ammonia.py; merge screens classes
Selection update callback (draft)
Add debounce
Reinit screen only at selection change
Update selection screen

Handle selection window movements.
Fix syntax error; fix welcome screen interactions
Add DEBOUNCE constant
Add calibrate probe selection screen (draft)
Start a new update thread only if the screen_update method is present
Fix method name and Calibrate screen super class initialization
Remove useless Selection::screen_update method
Better debounce handling (draft)

See

 * https://learn.adafruit.com/tilt-sensor/using-a-tilt-sensor
 * http://arduino.cc/en/Tutorial/Debounce