Add Hd44780i2c driver (#173)

* hd44780i2c: add support for hd44780i2c LCD display.

hd44780i2c/registers.go

@@ -0,0 +1,44 @@
+package hd44780i2c
+
+const (
+
+	// commands
+	DISPLAY_CLEAR        = 0x01
+	CURSOR_HOME          = 0x02
+	ENTRY_MODE           = 0x04
+	DISPLAY_ON_OFF       = 0x08
+	CURSOR_DISPLAY_SHIFT = 0x10
+	FUNCTION_MODE        = 0x20
+	CGRAM_SET            = 0x40
+	DDRAM_SET            = 0x80
+
+	// flags for display entry mode
+	// CURSOR_DECREASE  = 0x00
+	CURSOR_INCREASE = 0x02
+	// DISPLAY_SHIFT    = 0x01
+	DISPLAY_NO_SHIFT = 0x00
+
+	// flags for display on/off control
+	DISPLAY_ON       = 0x04
+	DISPLAY_OFF      = 0x00
+	CURSOR_ON        = 0x02
+	CURSOR_OFF       = 0x00
+	CURSOR_BLINK_ON  = 0x01
+	CURSOR_BLINK_OFF = 0x00
+
+	// flags for function set
+	// DATA_LENGTH_8BIT = 0x10
+	DATA_LENGTH_4BIT = 0x00
+	TWO_LINE         = 0x08
+	ONE_LINE         = 0x00
+	FONT_5X10        = 0x04
+	FONT_5X8         = 0x00
+
+	// flags for backlight control
+	BACKLIGHT_ON  = 0x08
+	BACKLIGHT_OFF = 0x00
+
+	En = 0x04 // Enable bit
+	// Rw = 0x02 // Read/Write bit
+	Rs = 0x01 // Register select bit
+)