Fix specifying build targets

Hi all.  

Gave `grunt build` another tweak because it looks like 6bda2e0ec844057ef2b2ae625b33b9d3a0c82166 unintentionally borked how the build task was determining which platforms to build, so it just defaulted to building all of them as initially defined in the `nodewebkit.options` object.

Specifically, [Gruntfile.js#L216](https://github.com/mephux/komanda/blob/70cebe253bfcba9ab28069404cc8b0dfc3f0bd09/Gruntfile.js#L216) where the `nodewebkit` task now takes an array of platforms instead of using a series of keys [like it used to](https://github.com/mephux/komanda/blob/a6c03a90ce812ba8b383f9be73b8e5bd98e9048d/Gruntfile.js#L215-L218) is what threw everything off.

Also, curiously, the `nodewebkit` task no longer understands `'mac'` and now only understands `'osx'`, so I threw in some logic in the build task to handle that.  I did have to change the commands for [`grunt exec:createDmg`](https://github.com/OnAzureWings/komanda/blob/3914e03b52cd12e03a599421919a05896f041b53/Gruntfile.js#L241) and [`grunt shell:create_dmg`](https://github.com/OnAzureWings/komanda/blob/3914e03b52cd12e03a599421919a05896f041b53/Gruntfile.js#L291) because they use the path to the OS X build, so if someone who can run those tasks can test them for me?  `./dist/` is in the `.gitignore` and not part of the project.  I was hesitant to change too much in regards to the `mac`/`osx` discrepancy, but I can go back through and change everything else over if it's desired.

Cheers.