Fix gpxexporter for python 2.6

The extend method has been added in python 2.7 . Use the append
method in a loop instead to be backward compatible.

Debian 6 e.g. still has python 2.6.6

Change-UUID: c00e3aa1fd2d41f2b953fd4058482d39
Merge pull request #6 from sercxanto/gpxexporter

Fix gpxexporter for python 2.6