Add Places Autocomplete functionality

This pull request adds Places Autocomplete functionality to this library. It includes a new method on the `GooglePlaces` class: `autocomplete`, a new new `GoogleAutocompleteSearchResult`, as well as a new `Prediction` class to handle responses from the autocomplete endpoint. Finally, it includes an additional passing unit test (with fixture data).

Here's a quick demo setup:

```
from googleplaces import (
    GooglePlaces,
    types,
    lang
)

API_KEY = <your api key>

google_places = GooglePlaces(API_KEY)

# query_result = google_places.autocomplete(
#         input='Cuba',
#         language=lang.SPANISH,
#         types=types.AC_TYPE_CITIES,
#         location=dict(lat=40.7310523, lng=-73.99348)
#         )

# query_result = google_places.autocomplete(
#         input='Hattie',
#         location=dict(lat=36.1528232, lng=-86.7838999)
#         )


# nearby search demo
# query_result = google_places.nearby_search(
#         location='Nashville, TN', keyword='Fish and Chips',
#         radius=20000, types=[types.TYPE_FOOD])

```
Thanks for this refactoring :-)