Cryptocat node-webkit (Windows, Mac OS X, Linux desktop apps)

I feel like my project to wrap Cryptocat in node-webkit is coming to an end. :)

The auto-update procedure can be tested [with these builds](https://github.com/majodev/cryptocat/tree/master/src/node-webkit#let-me-test-this-with-some-prebuild-binaries). TL;DR: I've made various modifications/injects to make sure the apps behave like a desktop app and written some grunt magic to simplify the release/deploy step for these kind of apps. Please take a look at my [Contribution Keypoints](https://github.com/majodev/cryptocat/tree/master/src/node-webkit#contribution-keypoints).

Speaking of deploys, after this is merged, you'll want to:
* generate your own DSA keys ([see here](https://github.com/majodev/cryptocat/tree/master/src/node-webkit/dsa)) to sign the zipped updates.
* find a secure place to host the zipped Cryptocat binaries
* find a secure place to host the `package.json`, which provides the update manifest data
* modify `REMOTE_UPDATE_DIR` in the [`Gruntfile.js`](https://github.com/majodev/cryptocat/blob/master/src/node-webkit/Gruntfile.js#L26) to point to the right binary folder.
* Eventually: modify the file-naming-pattern of the zipped apps ([here](https://github.com/majodev/cryptocat/blob/master/src/node-webkit/Gruntfile.js#L65), [here](https://github.com/majodev/cryptocat/blob/master/src/node-webkit/Gruntfile.js#L160) and [here](https://github.com/majodev/cryptocat/blob/master/src/node-webkit/Gruntfile.js#L211))

Any feedback on the code/documentation welcome.

Fixes #566