#49 Show link to the marker on the map in the e-mail notification

See the initial request on issue 36

I did this on my local version of wifi-with-me. Some results with the good old pdb:

(Pdb) context
{'contrib_url': 'http://example.com/wifi/map/#6', 'contrib': <Contrib: #6 test>}

(Pdb) body.render(context).encode("utf-8")
b'Nouvelle demande de la part de test\n\n\xc3\x80 retrouver sur http://example.com/wifi/map/#6\n\nBien \xc3\xa0 toi,\n\nWifi-with-me\n'

@jocelyn: Is it more or less what you expected?

Merging, because itworks® :).

However, when #50 will be merged, we would better use the get_absolute_url() method it introduces.

Thanks !