Add support of reading WMTS Get Cap document

This commit fixe #2721. It still lacks the possibility to create layer from the GetCap document. It also lacks the reading of TMS limits.
This can't work in advanced mode :

In `TMS_PARSERS_` you set the identifier as

    'Identifier': ol.xml.makeObjectPropertySetter(
              ol.format.XSD.readString, 'identifier')

You tells the compiler to set this property name explicitly to 'identifier' but when you call the property via `value.identifier` the compiler renames it so the reference is lost.