Add g:GPGRecipientsMenu

Allow to set initial contents for recipients menu with
g:GPGRecipientsMenu. Example usage:

let g:GPGRecipientsMenu=""
    \."GPG: Example User <example@example.com>\<NL>"
    \."GPG: Other User <otherexample@example.com>\<NL>"
Refactor g:GPGRecipientsMenu to use list and rename to g:GPGPossibleRecipients

  let g:GPGPossibleRecipients=[
    \"Example User <example@example.com>",
    \"Other User <otherexample@example.com>"
  \]
Merge pull request #21 from tyll/menu

Add g:GPGRecipientsMenu