Generate source map of minified ol.js

Allows automatic unminification by the browser.

A postprocessing is required to:
- fix the paths to the sources;
- add the link from the minified script to its map.

If someone knows how to do the first item in Python or nodejs, please provide a
patch. A simple shell script is:
```
\#!/bin/sh
pwd=`pwd`
sed -i "s!$pwd/build!olsource!g" 'build/ol.js.map'
sed -i "s!$pwd!olsource!g" 'build/ol.js.map'
```

The second item should always be manually handled to avoid 404 errors
in debuggers
`echo '//# sourceMappingURL=ol.js.map' >> build/ol.js`

Finally, create an alias / copy ol3 directory to /olsource in your
deployed environment.

config/ol.json

@@ -61,6 +61,8 @@
     "compilation_level": "ADVANCED",
     "warning_level": "VERBOSE",
     "use_types_for_optimization": true,
-    "manage_closure_dependencies": true
+    "manage_closure_dependencies": true,
+    "create_source_map": "build/ol.js.map",
+    "source_map_format": "V3"
   }
 }