Use a submodule for openlayers

This is to work around a problem in ol3: https://github.com/openlayers/ol3/issues/2668. When the problem is fixed we will need to decide how we want to install ol3 in ngeo, with npm or as a submodule.
Merge pull request #40 from elemoine/openlayers

Use a submodule for openlayers
Fix a bug in Makefile