Support for OpenStreetMap XAPI

This branch adds support for OpenStreetMap XAPI http://wiki.openstreetmap.org/wiki/Xapi to OpenLayers-3. It defines two new classes:

ol.parser.OSM - parses the response from the XAPI and creates features out of the elements (currently only 'nodes' tesed)
ol.source.XAPI - creates new data source (based on ol.source.vector), which takes url, type and aditional parameter as arguments

besides those two files, tests and xapi-layer example are included