Framework for serializing structures to XML

This adds several helper functions for serializing to XML:

* ol.xml.serialize: Counterpart to ol.xml.parse. By splitting
  the serialization process up into a node factory and a node
  writer, note writers can easily be used for different
  namespaces.

* ol.xml.pushSerializeAndPop: Counterpart to
  ol.xml.pushParseAndPop.

* ol.xml.makeStructureNS: Works like ol.xml.createParsersNS,
  but works for arbitrary structures.

* ol.xml.makeChildAppender: If the top item of the stack has
  the new ol.xml.NodeStackItem type, this helper function
  can be used to create a serializer that appends the current
  node to its designated parent.

* ol.xml.makeChildNodeFactory: Creates a node factory which
  produces child nodes from an array of node names which are
  passed to ol.xml.serialize.

* ol.xml.makeSequence: A convenience function for creating
  xsd:sequence structures. Takes an object literal and an
  ordered list of the keys, and returns an array that can be
  passed as values to ol.xml.serialize.

* ol.xml.makeSimpleTypeWriter: Using e.g. the new write*TextNode
  functions from ol.format.XSD, this function creates a node
  writer that writes simple type nodes for values like strings
  or numbers.

The following commits will be using this new framework for
implementing ol.format.GPX.writeFeatures, and prose
documentation with instructions based on what was said above
will be added.
Added wpt serialization for ol.format.GPX