Accept StringIO and TempFile

[Qwertie-]

This line checks if the input is of the the type File. It would be helpful if it also accepted Tempfile and StringIO so you could give it the result of open-uri to load gpx files from http.

[tyrauber]

This issue is resolved here.

To read a GPX file from a string, use :gpx_data.
gpx_file = GPXFile.new(:gpx_data => '<xml ...><gpx>...</gpx>')

The Readme should be updated to reflect this.