Add signing support

[pixlcrashr]

It is a necessary feature, that you can sign a .pbo file within this package, because currently .pbo signing is just available on windows (because of an .exe) and unix-based-systems via wine.

The file signing implements a simple private/public key system. The methods used are RSA 1 & 2 as listed here: https://community.bistudio.com/wiki/DSKeys