Get 'Tweet' and 'Google +1' buttons back

The sharing widget is still using the old-style configuration keys **'twitter_tweet_button'** and **'google_plus_one'**, which don't exist anymore.

Changing them to **'twitter.tweet_button'** and **'googleplus.plus_one'** works.

PS: I also renamed **'tweet-button'** to **'tweet_button'** considering the naming convention.