Fix File/Classname inconsistencies so the class can be autoloaded on case-insensitive filesystems

The main class was called BENCODE but was in a file called Bencode.php. This is fine on systems with case-insensitive filesystems (Windows, Mac OSX) but results in 'PHP Fatal error:  Class 'Bhutanio\BEncode\BENCODE' not found' on Linux and I presume other systems with case-sensitive filesystems.

Renaming the class and filename solves this issue and allows the class to be autoloaded on Linux.