[OPEN] Update grunt build scripts for linux

Replaces `scripts/setup_linux_build.sh`

@timburgess @DaBungalow would you like to test out these updates? Someone else with committer rights will need to handle merging though.

These changes bring the Linux setup and build Grunt tasks up to par with mac and windows. This doesn't replace the one-line `wget` (see https://gist.github.com/jasonsanjose/5514813) required for downloading the git repositories and installing Node.js and Grunt.

Here are the highlights:

* I've hosted CEF binaries in using GitHub's release feature https://github.com/jasonsanjose/brackets-shell/releases/tag/cef-builds. These binaries are modified from the original zip files. To reduce the download size, I've removed the `Debug` folder. For folks like @timburgess that are doing work with the native shell, it won't be too hard for them to get the full binaries manually.
* There's some extra commits from another outstanding pull #286. The prior pull should be merged first.
* CEF and Node.js downloads are supported for 32 and 64 bit. I haven't tested the build process on 64-bit yet.
* Speaking of build, you should be able to just pull this branch and run `grunt` to setup and build. `grunt installer` will produce the debian package
* There's a new `postinstall` hook after `npm install` to install the compile-time dependencies 
* Incremental linux builds can be done via `grunt build` or simply `make` as before