[tools-amd/en] add AMD / require.js tutorial

This adds a very basic tutorial for JavaScript's AMD spec and require.js, targeted at people using JavaScript on a daily basis.

Since English is not my native language feel free to point out the most awkward phrasings and other mistakes.
Technical input is welcome, too.

On a side note: is it possible to export only a portion of the code blocks included in the document when doing "Get the code"?
I found it rather hard to produce a running example of code when having to explain how to separate code into several files...
If I could only export the first block it would be running just fine, but right now I'd had to resort to using a jsfiddle. 

I'd guess this is probably a problem for other "tools" documents, too.
@levibostian While manual linebreaks won't hurt at the moment (should we @-mention Linus now?), it looks like it's not really necessary.

Compare https://raw.githubusercontent.com/adambard/learnxinyminutes-docs/master/coffeescript.html.markdown against http://learnxinyminutes.com/docs/coffeescript/

The 80 char rule should only apply to code as it's `white-space` behavior is set to `pre` by the UA stylesheets. See: https://developer.mozilla.org/en-US/docs/Web/CSS/white-space