Add support for Video Website Cards

[juanshishido]

• change API_VERSION to 2
• add VideoWebsiteCard class
• add video_website_cards method under the Account class

[jbabichjapan]

I'm not sure if this is needed because the other card formats also use just the RESOURCE_COLLECTION?

[juanshishido]

My understanding is that including this under the Account class let's us retrieve the video website cards that belong to the particular account.

For example:

In [3]: client = Client(CONSUMER_KEY, CONSUMER_SECRET, ACCESS_TOKEN, ACCESS_TOKEN_SECRET)
   ...: account = client.accounts(ACCOUNT_ID)
   ...: 

In [4]: vwcs = account.video_website_cards()

In [5]: type(vwcs)
Out[5]: twitter_ads.cursor.Cursor

In [6]: vwcs.first.id
Out[6]: u'455hx'

In [7]: vwcs.first.card_uri
Out[7]: u'card://860983079292198912'

In [8]: vwcs.first.card_type
Out[8]: u'VIDEO_WEBSITE'

You can do this type of thing for any method (entity) defined under the Account class.

[tushdante]

LGTM!