Mac DMG build should embed SHA somewhere

[peterflynn]

(Split from #1524 per Peter T)

Currently, when we build an installer there's no record of what Git SHA the bits were pulled from. We have to keep track of this manually if we want any record of it (e.g. by tagging and being careful to always generate installers from a fixed tag, and remembering which installers go with which tags).

It'd be much better to have the staging scripts actually create a file on disk that is included in the installer payload, containing a record of which brackets & brackets-shell SHAs were used.

[gruehle]

@jasonsanjose - would you mind regressing by verifying in tomorrow's daily build? Thanks!

[jasonsanjose]

Will do.

[jasonsanjose]

Confirmed in build 0.14.0-3717 (/Brackets_Builds/2012-09-11_14-27-44). package.json:

{
    "name"                  : "Brackets",
    "version"               : "0.14.0-3717",
    "homepage"              : "http://brackets.io",
    "issues"                :
    {
        "url"               : "http://github.com/adobe/brackets/issues"
    },
    "repository"            :
    {
        "type"              : "git",
        "url"               : "https://github.com/adobe/brackets.git",
        "branch"            : "master",
        "SHA"               : "4361763dcba28933111491250650a06de0cad65e"
    },
    "config"                :
    {
        "about_icon"        : "styles/images/brackets_icon.svg",
        "show_debug_menu"   : true,
        "enable_jslint"     : true,
        "forum_url"         : "https://groups.google.com/forum/?fromgroups#!forum/brackets-dev"
    }
}