added namespace support and PS-0 compatible autoloader

I have wrapped the existing codes into "Abraham\TwitterOAuth" namespace and structured it in a PS-0 compatible fashion. I have also included an autoloader to demonstrate the auto loading compatibility. 

The library can be now ported to Composer/Packagist eco system and can be dropped directly into a Symfony2 "src" directory and used without modification. 

If this pull request is accepted, I shall create a composer package out of it as well - so that people could install it as a project dependency easily.