Add script to update the `server-content` branch

Provide script that will allow developers to easily run the build
process and place its result into a branch from which the server
can just pull periodically, thus, not needing to do the build itself.

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Issue background:

 Having the server do the build step has some downsides:

  * duplicate setup (on the developer's machine, as well as the server)
  * sometimes because of the outdated software on the server, updating
    or installing new components can be difficult/time consuming (see:
    #107, #116)

 Because of the things mention above and because developers usually
 do the build step on their machines anyway, we decided to move to just
 having the developer execute an `update` script that will run the build
 step and copy the resulting code in a branch called `server-content`,
 branch from which the server can just pull periodically.

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Close h5bp/html5boilerplate.com#116.
Add `grunt-uncss` plugin

This change significantly reduces the size of the final style
sheets, by removing all the unused CSS.

E.g.: for `index.html`, the resulting `main.css` file will be
up to ~30% smaller:

    before:         7182 B
    after:          4984 B
    ──────────────────────
    reduction:      2198 B [30.60%]

Thanks @XhmikosR

Ref   https://github.com/addyosmani/grunt-uncss.
Close h5bp/html5boilerplate.com#114.