[WIP] Option to save comments in git note on merge

Option to save comments in git note on merge in the same way symfony is doing this.

The option can be used with the command

    gush p:merge --org=myname --repo=myrepo 123 --add-comments --remote=myremote

And the output will look something like:

````
---------------------------------------------------------------------------

by peterrehm at 2014-01-29T10:41:42Z

Test

````

I don't like the way the notes have to be actually attached (with the several git commands) but I could not find
any other way.