Add BufferedImage and Fixed Text support to Captcha image generation

- added ImageProducers and Renderers to allow adding BufferedImage to anywhere in the captcha image
- added FixedStraightLineNoiseProducer to add straight lines any where on the captcha image
- added some Text Producers and Renderers to allow drawing text anywhere on the capcha image
- updated ImageUtil with resize and load image functions

@sinofool