readshapefile returns a multipolygon as several polygons

I wanted to draw a choropleth map using basemap. My plan was to read the polygons from the shapefile vis `readshapefile` and then further process them using `shapely`, `descartes`, etc.

The problem that I encountered was that `readshapefile` returned multiple polygons for each `MultiPolygon`. This causes confusion, as for the enclosed polygon we will have more than one record.

I think the fix will be to return a `MultiPolygon` instead of multiple polygons.

Here is an example:

``` python
from collections import OrderedDict
import fiona
from mpl_toolkits.basemap import Basemap

rec1 = dict()
rec2 = dict()

rec1['id'] = 0
rec1['type'] = 'Feature'
rec1['properties'] = OrderedDict([(u'NAME', 'outer'),(u'VALUE', 100)])
rec1['geometry'] = dict()
rec1['geometry']['type'] = 'MultiPolygon'
rec1['geometry']['coordinates'] = [[[(-50,40),(50,40),
(50,-40),(-50,-40),
(-50,40)
]],
[[(-45,35),(45,35),
(45,-35),(-45,-35),
(-45,35)
]]]

rec2['id'] = 1
rec2['type'] = 'Feature'
rec2['properties'] = OrderedDict([(u'NAME', 'inner'),(u'VALUE', 1)])
rec2['geometry'] = dict()
rec2['geometry']['type'] = 'Polygon'
rec2['geometry']['coordinates'] = [[(-45,35),(45,35),
(45,-35),(-45,-35),
(-45,35)
]]
driver = 'ESRI Shapefile'
crs = {'no_defs': True, 'ellps': 'WGS84', 'datum': 'WGS84', 'proj': 'longlat'}
schema = {'geometry': 'Polygon',
'properties': {'NAME': 'str',
'VALUE': 'float:15.2'}}

with fiona.open(
'/tmp/foo.shp',
'w',
driver=driver,
crs=crs,
schema=schema) as c:
c.write(rec1)
c.write(rec2)

m = Basemap()
m.readshapefile('/tmp/foo','foo')
for info,region in zip(m.foo_info, m.foo):
print info
print region
```

for which the output is:

```
{'RINGNUM': 1, 'NAME': 'outer', 'VALUE': 100.0, 'SHAPENUM': 1}
[(-50.0, 40.0), (50.0, 40.0), (50.0, -40.0), (-50.0, -40.0), (-50.0, 40.0)]
{'RINGNUM': 2, 'NAME': 'outer', 'VALUE': 100.0, 'SHAPENUM': 1}
[(-45.0, 35.0), (-45.0, -35.0), (45.0, -35.0), (45.0, 35.0), (-45.0, 35.0)]
{'RINGNUM': 1, 'NAME': 'inner', 'VALUE': 1.0, 'SHAPENUM': 2}
[(-45.0, 35.0), (45.0, 35.0), (45.0, -35.0), (-45.0, -35.0), (-45.0, 35.0)]
```

Start at the Basemap.readshapefile entry point and reproduce the supplied Fiona example, comparing returned records with the source geometries. Done when the MultiPolygon case preserves one source feature's geometry and metadata without breaking the separate Polygon case.

Written by the indexing model from the issue text.