Add gitattributes to reduce unnecessary files from composer installs.

When using the composer argument `--prefer-dist`, git archives will be used. This reduces files that are typically useless when using phpCAS as a library, as opposed to developing it. When using phpCAS as a library, and deploying, tests and raw documentation are unnecassary. When developing phpCAS, the preferred method is a git clone, and will continue to work properly.