Add support for signed phars with signature attached

[theseer]

Apart from having detached signature files foo.phar.asc for a given foo.phar, gpg also supports signing the archive and keep the signature attached (usually stored as foo.phar.gpg).

We might want to support this way of distribution. An example using this means of distribution can be found with wp-cli (https://github.com/phar-io/phar.io/pull/47).