Adding the option to download GPG signatures

[Travis McPeak]

Hi all,

I'm beginning to work on a scheme to verify signatures for PyPI packages.  To that end, I'd like to add the option to pip to download them.

I could see this structured as a separate download option:

   "pip install <package> -ds ." (as an alternative to the -d option)

Or maybe as an additional flag:

   "pip install <package> -d -s ." (as an additional flag to the -d option)

I'm willing to work on this myself, I just wanted to run it by the folks here before I begin working on it.

Thank you,

 -Travis

[Travis McPeak]

Thoughts?