OAuth2 support

[wetneb]

Since fairly recently, Wikimedia wikis support OAuth 2, which has some advantages over OAuth 1. For instance, it is possible to use it without a client secret, making it interesting for open source local clients such as OpenRefine.

We could add support for OAuth 2 in Wikidata-Toolkit. This could consist in a third subclass of ApiConnection perhaps, or the extension of our existing subclass for OAuth. The library we use for OAuth 1 support, scribejava, seems to also support OAuth 2.