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.
yeah, perhaps it would be better to shuffle this around. It might be better to start with the hosted examples on ol3.org where the requires are split off from the code, and then use examples/* for a later example of compiling in with the code. Originally, I thought that people would compile their code together with the library, but I've changed my mind; there are simply too many complicating factors, and I now assume that most people will just want a stand-alone build customised to their app(s).