Proxies: accept as string

[KostyaEsmukov]

I think that accepting a single string for the proxies param would be great. The urllib2-style dict objects with schemes might be confusing to the users of geocoders.

# Current usage:
geolocator = Nominatim(scheme='https', proxies={'https': 'http://192.0.2.1:8080'})
# Proposed usage:
geolocator = Nominatim(proxies='http://192.0.2.1:8080')

#288 should be merged first.