GML parser.

This adds a parser (read/write) for GML v2 and v3. GML v3 is limited to the
simple features profile of GML 3.1.1, just like OpenLayers 2 was. This will
be the basis for the WFS parser, but it only makes sense to continue this work
once feature modification (insert, update, delete) is in place in ol3. So the
WFS parser will be another pull request.
incorporate review from @ahocevar
do not specify the source projection in ol.parser.ReadFeaturesOptions anymore
Merge pull request #709 from bartvde/gmlfresh

GML parser (r=@ahocevar)