Support for SUPublicDSAKeyFile

Originally: "As an alternative to supplying the public DSA key in the Info.plist file, it would be good to be able to supply the key in a file whose name is given in the Info.plist file (if it had a non-default file name). Example:
{{{
Info.plist:
    <key>SUPublicDSAKeyFile</key>
    <string>dsa_pub.txt</string>
}}}
where dsa_pub.txt is a file in the application's resource folder (Contents/Resources).

You may ask why? Well, I am having trouble adding the key file by means of CPP preprocessing of my Info.plist file, mainly because the key contains newline characters which are a pain to add by means of a preprocessor macro. The best I can get is a string with "\n" characters or without newline characters, neither of which works.

Also, the Info.plist file should be kept simple and short. Complex data should be elsewhere."

AMM: I think this will help adoption of DSA signatures.

This ticket was migrated from the old trac: re #57
Originally reported by: Eric Roller