Add get_printer_class function and new global constants to package-level scope.

Author: tdicola

adafruit_thermal_printer/__init__.py

@@ -0,0 +1,20 @@
+from adafruit_thermal_printer.thermal_printer import JUSTIFY_LEFT, \
+JUSTIFY_CENTER, JUSTIFY_RIGHT, SIZE_SMALL, SIZE_MEDIUM, SIZE_LARGE, \
+UNDERLINE_THIN, UNDERLINE_THICK
+
+def get_printer_class(version):
+    """Retrieve the class to construct for an instance of the specified
+    thermal printer version.  Pass in the printer firmware version as a numeric
+    value like 2.68, 2.64, etc.  You can get this value by holding the button
+    on the printer as it powers on and a test page is printed--look at the
+    bottom of the test page for the version number.
+    """
+    assert version is not None
+    assert version >= 0.0
+    if version < 2.64:
+        import adafruit_thermal_printer.thermal_printer_legacy as thermal_printer
+    elif version < 2.68:
+        import adafruit_thermal_printer.thermal_printer_264 as thermal_printer
+    else:
+        import adafruit_thermal_printer.thermal_printer as thermal_printer
+    return thermal_printer.ThermalPrinter