caca_export_string

to allow third-party applications to link with caca-sharp, and tune a
few things here and there.

* Destroy php resource in caca_file_close
* Improve sample program render.php

* Add php bindings for caca_get_dither_antialias_list, caca_render_canvas
* Add a new sample program examples/render.php

* caca_dithering_bitmap can load palette from gd resource
* Remove caca_set_dither_palette_gd function
* Adapt sample program "dithering.php"

* caca_get_event return false if type == CACA_EVENT_NONE otherwise return a new
  php resource
* Adapt sample program demo.php