Support PKCS#12 cert stores in PharPackageTask.

Allow a PKCS#12 certicate store to be used instead of a regular PEM-encoded
private key in the <pharpackage> task.
Usage is similar to that of private keys, with the "keypassword" attribute
being used as the store's passphrase.
Merge pull request #229 from fpoirotte/pkcs12

Support PKCS#12 cert stores in PharPackageTask.