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.
The last sentence may make the reader think that "there's a long learning curve" in trying to create custom builds. I like your description but I don't want to scare users too much :-)