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 can add that if you like. Dynamic module loading is an entirely separate issue, which I plan to look into once it's clearer what the compiler will provide. There is now also a `goog.module('x')`, similar to AMD's module definition, but it's not clear to me at the moment how that fits in with the current `goog.module` namespace.