Grunt (snort bellow) build tasks

`grunt` to build your extensions for development/deployment!

`grunt`, then load your extension for development or package for deployment from `temp/ext/ExtensionFolder`

* **default** does all of the following:
* **listfiles** creates a listing of assets (not specific to browser) in `temp/ls/` by file type (JS, CSS, text/image resource)
* **manifests** regenerates the browser-specific extension manifests from `manifests/` into `temp/manifests/` by extension, filling in extension name, version, and assets
* **copy** copies all the general assets, browser-specific assets, and manifests into `temp/ext/`

Tested on Chrome, OperaBlink, Safari, and Firefox

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In order to support "lightweight RES development" (i.e. people who don't want to install grunt and the required modules), I'll also modify makefile to hardlink from that same build folder, `temp/ext/`.  There is a small support costs: committing the built manifests to git after changing `manifests/**` or adding new files. Perhaps we can add a post-commit (post-merge?) hook on github to warn about "add/change without updating manifest".

Partially address https://github.com/honestbleeps/Reddit-Enhancement-Suite/issues/853