Add basic reading support for OWS Capabilities documents.

This PR adds a new format class for reading OWS Capabilities document. Tests are mostly taken from [old wmts parser test](https://github.com/openlayers/ol3/blob/master/old/test/spec/ol/parser/ogc/wmtscapabilities_v1_0_0.test.js).

The ol.format.OWSCapabilities is meant to later be used in ol.format.WMTSCapabilities.

Cheers.