Fix issue with JMagick Spi Providers

Hello and thank you for your work.

Please find a few fix I did to the SPI providers to use with a web application under JBoss AS 7.
* There was a class loader issue with the JMagickImageReaderSpiSupport.AVAILABLE contant. So I had to specify the class loader to use
* The TargaImageReaderSpi was causing errors when trying to seek a stream provided by a file upload. I so catched the exception.
* I changed the JPEGImageReaderSpi to handle Photoshop 3's JPEG, although they may not be a proper standard.

Regards,

Philippe