Add launchd plist for server daemon to Emacs formula

Since emacs version 23.1 the emacs executable accepts a --daemon flag which will start emacs as a server daemon. You can then connect to the server using emacsclient (reducing startup time etc).

This pull request adds a launchd plist to the Emacs formula that will allow to start the emacs daemon via launchd (e.g. at login).
See [the emacs wiki article](http://www.emacswiki.org/emacs/EmacsAsDaemon#toc7).

The following caveat will be output after installing emacs:
```
==> Caveats
To have launchd start emacs at login:
    ln -sfv /usr/local/opt/emacs/*.plist ~/Library/LaunchAgents
Then to load emacs now:
    launchctl load ~/Library/LaunchAgents/homebrew.mxcl.emacs.plist
```
This may be a little confusing for the user, but as of now I don't see any way to specify a different name for the caveat (like `To have launchd start the emacs daemon at login:`).