Build 3 phar releases.

We now build 3 separate phar archives:
- .phar = uncompressed
- .phar.gz = compressed with zlib
- .phar.bz2 = compressed with bz2

The signature algorithm has been upgraded from sha1/openssl to sha256.
The openssl signature mechanism has been removed entirely as we are gradually
moving to a GPG-based signature for authenticity/integrity verifications.