Grunt

This ha been difficult and it's not as complete as I would like but anyway it's a good step forward:

1- Enable gzip
2- Remove `connect-assets`
3- Move assets/css & assets/js to public
4- Add grunt and change start script

**grunt production**:

- Compile stylus files of all local css + vendor css( only files that do not have any `url()` expression because it would create problems with images path)

- Combine & compress all js files in a single one

**grunt development**

- As above but do not concat/minify js files

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The server can be start with **grunt start** (or **grunt start:production**) which run nodemon like before but with the right grunt task (development or production).

When nodemonn restarts it seems that express gives an error about address already in use (I think the process isn't killed correctly) but works well anyway. Maybe it would be a good idea on production not to use nodemon, but I think you don't use it already,

**grunt start:production** run only **grunt production**, do not set `NODE_ENV` to `production`, it should be set manually.

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`res.locals.habitrpg` in static pages had NODE_ENV always set to `production`, I had to change it to get this working. There is a comment about not showing server debug infos on static pages but anyway it will be set to `production` when the whole server is running with NODE_ENV set to production, so i think there aren't problems with that.

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**What is missing**

- compression and concatenation for css vendor files (and habitrpg-shared) with relative paths to images. Ideas are welcomed
- etag/file names with hash so we can set a far `maxAge` for `/public` files.  Ideas are welcomed here too


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These are big changes, please test it and report anything that doesn't work. Someone should test it on windows/mac to see that **grunt start** is working.