Do not allow packagist to pull non-PHP files

The way how Packagist work is to ask Github to export tag or branch as a archive, cache it locally and deliver to the user.

The problem here that the source tree might contains some non-essential scripts, tests and developer files that at minimum pollute ./vendor directory of the project, or even might cause issues (for example it might pull symlinks, that will not work because they point to git submodules).

The solution here is to use "export-ignore" tag for some paths, so that Github will not export non-essential and problematic files and directories, so that Packagist (and Composer) will not need to deal with them after all.

Links:

• .gitattributes reference: https://git-scm.com/docs/gitattributes#_creating_an_archive
• discussion on packagist issue tracker: https://github.com/composer/packagist/issues/270