Added GPX export support for run activities.

After your reply to my tweet (https://twitter.com/OrenNachman/status/549053425262927874) I discovered unBand. I noticed that you published source code and unBand was already able to download advanced data for GPX export. So I only implemented advanced data downloads (MapPoints) for run events and GPX export. For more info please contact me on twitter via DM: @ProH4Ck