Issue #16 Sixel supports

This PR is a quick implementation of SIXEL graphics(Issue #16).

It introduces the following new options.

```
-sixel2 : emit monochrome sixel
-sixel16 : emit 16-color sixel
-sixel256 : emit 256-color sixel
```

Note that we can get more beautiful picture using palette re-definition sequence If this software can do dynamic-palette quantization method(such as median-cut), but I didn't know how to do it.

### DEMO

- mlterm (it works with X11 / frame buffer / win32 version, http://mlterm.sourceforge.net)
![mlterm](https://f.cloud.github.com/assets/1162739/2239820/a666ed56-9c5a-11e3-9da4-f877e95f7497.png)

- xterm with "-ti vt340" option (color palette is limited up to 16)
![xterm-ti340](https://f.cloud.github.com/assets/1162739/2239823/e525bf90-9c5a-11e3-9f9c-1825e506abe5.png)

- RLogin (Japanese high spec terminal emulator. it also implements ReGIS. see http://nanno.dip.jp/softlib/man/rlogin/#REGWIND)
![rlogin](https://f.cloud.github.com/assets/1162739/2239824/22a3f83c-9c5b-11e3-91d9-a2e5480adaeb.png)

- tanasinn (A firefox extension, see http://zuse.jp/tanasinn/)
![tanasinn](https://f.cloud.github.com/assets/1162739/2239826/903ae7e8-9c5b-11e3-84b6-59bba4fa3d04.png)

I tested this patch on above 4 terminals.