Add support for different screen sizes (SSD1306 only) - fixes #23

Note: the SSD1306 chipset is only supported presently - should there be
a need for SH1106 support, that will be addressed later.

oled/device.py

@@ -124,6 +124,11 @@ def __init__(self, serial_interface=None, width=128, height=64):
             self.height = height
             self._pages = self.height // 8
 
+            # FIXME: Delay doing anything here with alternate screen sizes
+            # until we are able to get a device to test with.
+            if width != 128 and height != 64:
+                raise ValueError("Unsupported mode: {0}x{1}".format(width, height))
+
             self.command(
                 const.DISPLAYOFF,
                 const.MEMORYMODE,
@@ -193,10 +198,20 @@ def __init__(self, serial_interface=None, width=128, height=64):
             self._buffer = [0] * self.width * self._pages
             self._offsets = [n * self.width for n in range(8)]
 
+            # Supported modes
+            settings = {
+                (128, 64): dict(multiplex=0x3F, displayclockdiv=0x80),
+                (128, 32): dict(multiplex=0x1F, displayclockdiv=0x80),
+                (96, 16): dict(multiplex=0x0F, displayclockdiv=0x60)
+            }.get((width, height))
+
+            if settings is None:
+                raise ValueError("Unsupported mode: {0}x{1}".format(width, height))
+
             self.command(
                 const.DISPLAYOFF,
-                const.SETDISPLAYCLOCKDIV, 0x80,
-                const.SETMULTIPLEX,       0x3F,
+                const.SETDISPLAYCLOCKDIV, settings.displayclockdiv,
+                const.SETMULTIPLEX,       settings.multiplex,
                 const.SETDISPLAYOFFSET,   0x00,
                 const.SETSTARTLINE,
                 const.CHARGEPUMP,         0x14,