Made repo psr-0 compliant for Composer

The integration for Composer wasn't done quite right. I removed AvaTax.php from classes and made the functions there public static. Also the autoload register is not needed since Composer takes care of this. Creating a raw function in global space for libraries may cause name conflicts. I put all the classes in an 'AvaTax' namespace as per psr-0 specs. Had remove the 'class' word from filenames for Composer to properly autoload the classes.