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.
olx.js shouldn't be used as an externs file when compiling the application and ol3 together. It should be used as a regular input file. Using it as an externs file works, but many properties that could be renamed won't be renamed.