Custom build tutorial

Long promised ...

I've included an amended version of the introduction tutorial, which should help with #2437. The references to the hosted builds will have to be changed if and when these move to openlayers.org.

Unfortunately, I happened to use heatmap-earthquakes example, which has a problem #2477. The tutorial will need changing to handle the changes in this PR.

I've not changed the index for the moment, as I have another 'exploring the api' tutorial coming up, and will probably suggest some restructuring when I submit that.
I'm assuming that devs are more likely to be familiar with Node and other systems than with Closure, so think they need to know the fundamental ways they differ