Implement GPX parser (read/write).

Implement a parser for GPX which supports read and write.
address review by @fredj
make feature info work on touch devices for gpx example see #727
Fix XML serialize and xml_eql in IE9

Use the xml property instead in IE for serializing XML.
The xml_eql tests were failing in IE9 on a difference between null and an
empty string for namespaceURI of attributes, this difference is not
relevant to our testing so make sure we ignore.
Static read and write functions on WKT parser

Use a singleton to implement static read and write methods on ol.parser.WKT.
FeatureId support in GML (r/w) and KML (w)

Now that we have FeatureId support (see #733), we can add this back to the
GML parsers (v2 and v3). Also add write support for FeatureId in KML, read
support was already added by @fredj
Move squarifiedRatio into private static function
Move makeMBR into private static function
Make minWidth and maxWidth private member variables
Move more functions into private methods
Move search and searchReturningObject into methods
Move remove into method
Move insert into method
Clean up some comments
Sort methods alphabetically
Add ol.extent.empty
Use empty extents and refactor for clarity
Sort exported properties
Export constrainResolution and constrainRotation from ol.View2D
Merge pull request #741 from fredj/export-view-constraints

Export constrainResolution and constrainRotation from ol.View2D