Rename rc.cartopy-->rc.geo and updtae/improve Proj

This commit renames rc['cartopy.circular'] to rc['cartopy.round'] to
match the basemap 'round' keyword, renames rc['cartopy.autoextent']
to rc['cartopy.extent'] with optional values 'globe' and 'auto' (the
old 'cartopy.autoextent' boolean is translated to these values), and
moves application of 'extent' and 'round' from GeoAxes.__init__ to
GeoAxes.format, permitting toggling after creation and permitting
loose passage to top-level pplt.subplot and pplt.subplots commands.
It also improves up Proj by applying 'geo.round' to basemap projections,
interpreting 'latlim' as 'min_latitude' and 'max_latitude' for cartopy
Mercator projections (also translates 'lonlim' to 'min_longitude' and
'max_longitude' which for now raises error), renaming 'lon_0' and
'lat_0' to 'lon0' and 'lat0' with a silent deprecation that accepts the
former versions (this is more consistent other underscoreless keywords),
and updating the URLs from proj4.org to proj.org.