Initial implementation of support for AMD

* HighchartsAdapter is still being written directly to `window`
* The various parts of Highcharts can now be written as modules. This means
that the high*.debug.js files can be dropped, as the AMD implementation will
take care of dependency management. For production releases one would simply
concatinate all the dependencies together to get a single dist file. There are
tools which can detect dependencies and handle the bundling automatically, such
as require.js' optimizer, r.js.