Signature check operations for RFC3256 signed messages

Implemented `signed?`and `signature_valid?` operations for checking signatures on signed-only mails.

Implemented as discussed in [pull request #2.](https://github.com/jkraemer/mail-gpg/pull/2#issuecomment-27772061), meaning that `signed? ` throws an error if called on `encrypted` mails.

For encrypted emails the signature should be verified as part of `decrypt`by setting the `:verify`option.

Added documentation section on this in *Verify signature(s)* section.

Note that this does not yet support `signature_valid?` for so called * Inline GPG* messages (added to todo list)