GnuPG email signing for Windows initial commit

[asctime]

Commit 622e65f offers straightforward patches to e-util/e-signature-utils.c and camel/camel-gpg-context.c using simple popen for preliminary support of gnupg signing. Requires installation of a working GnuPG package, such as this one. Some basic setup steps:

1. Start agent daemon with "gpgconf -v --launch gpg-agent" as evolution user
2. bootstrap user's gpg installation with "gpg -K"
3. Create first new key using "gpg --gen-key"
4. Get the long "Key ID" hash for your new key using "gpg --list-keys"
5. Enter this ID hash on the "Security" Tab of your Ev2W mail account configuration.
6. You should now be able to sign emails with your GnuPG key on Windows.