CentOS executes copied Source

CentOS by default executes any file containing an extension part with
`.php` in it. Thus also the `.php.txt` files that are used to show the
source code.

This commit adds a .htaccess file that disables that behaviour and as
such fixes https://github.com/phpDocumentor/phpDocumentor2/issues/1157#issuecomment-39615136
Merge pull request #27 from mvriel/hotfix/phpDocumentor2/1157

CentOS executes copied Source