Add a time named argument to to_png (#764)

* Add a `time` named argument to `to_png`

Since it might take longer than 3 seconds to render the map, this adds a time named arg that allows you to delay the screenshot.

* Fix lints, builtin clobber and docstring

* fix trailing whitespace

* one more :-(

folium/folium.py

@@ -267,8 +267,18 @@ def _repr_html_(self, **kwargs):
             out = self._parent._repr_html_(**kwargs)
         return out
 
-    def _to_png(self):
-        """Export the HTML to byte representation of a PNG image."""
+    def _to_png(self, delay=3):
+        """Export the HTML to byte representation of a PNG image.
+
+        Uses Phantom JS to render the HTML and record a PNG. You may need to
+        adjust the `delay` time keyword argument if maps render without data or tiles.
+
+        Examples
+        --------
+        >>> map._to_png()
+        >>> map._to_png(time=10)  # Wait 10 seconds between render and snapshot.
+        """
+
         if self._png_image is None:
             import selenium.webdriver
 
@@ -284,7 +294,7 @@ def _to_png(self):
                 driver.execute_script("document.body.style.width = '100%';")  # noqa
                 # We should probably monitor if some element is present,
                 # but this is OK for now.
-                time.sleep(3)
+                time.sleep(delay)
                 png = driver.get_screenshot_as_png()
                 driver.quit()
                 self._png_image = png