Initial commit of read_postgis function

Wraps read_sql() and looks for the specified geometry column which gets
converted to shapely geometries. It returns a GeoDataFrame.

Example:
```
df = GeoDataFrame.read_sql("SELECT id, geom, highway, oneway FROM roads LIMIT 5", con)
df
           id highway oneway                                           geometry
0   4317678        5  False  LINESTRING (-92.0983609999999970 31.4399659999...
1  11803224        3  False  LINESTRING (-92.3248040000000003 30.5609140000...
2   4285654        5  False  LINESTRING (-90.8962089999999989 30.9225339999...
3   4286812        5  False  LINESTRING (-90.9428329999999931 30.7880220000...
4  12057238        3   True  LINESTRING (-91.5078129999999987 30.3997999999...

```
`df` is a GeoDataFrame.

Not sure the best way to test this with travis.