Add testing for read_postgis function

It also installs postgis and creates a simple database filled with the borough
data. A new requirements.test.txt file is created so that not everybody
installing geopandas is required to install the packages for testing, psycopg2
in this case.

tests/test_geodataframe.py

@@ -7,6 +7,7 @@ import urllib2
 
 import numpy as np
 from shapely.geometry import Point, Polygon
+import psycopg2
 
 from geopandas import GeoDataFrame
 
@@ -82,3 +83,26 @@ class TestDataFrame(unittest.TestCase):
         lonlat = df2.to_crs(epsg=4326)
         utm = lonlat.to_crs(epsg=26918)
         self.assertTrue(all(df2['geometry'].almost_equals(utm['geometry'], decimal=2)))
+
+    def _validate_sql(self, df):
+        # Make sure all the columns are there and the geometries
+        # were properly loaded as MultiPolygons
+        columns = ('borocode', 'boroname', 'shape_leng', 'shape_area')
+        for col in columns:
+            self.assertTrue(col in df.columns, 'Column {} missing'.format(col))
+        self.assertTrue(all(df['geometry'].type == 'MultiPolygon'))
+
+    def test_read_postgis_default(self):
+        con = psycopg2.connect(dbname='test_geopandas')
+        sql = "SELECT * FROM nybb;"
+        df = GeoDataFrame.read_postgis(sql, con)
+        self._validate_sql(df)
+
+    def test_read_postgis_custom_geom_col(self):
+        con = psycopg2.connect(dbname='test_geopandas')
+        sql = """SELECT
+                  borocode, boroname, shape_leng, shape_area,
+                  geom AS __geometry__
+                  FROM nybb;"""
+        df = GeoDataFrame.read_postgis(sql, con, geom_col='__geometry__')
+        self._validate_sql(df)