core: Add a GDataOAuth2Authorizer for OAuth 2.0 support

This adds a new GDataAuthorizer subclass to support OAuth 2.0
authorisation, which is needed for the Google Tasks unit tests.

This includes support for all Google OAuth 2.0 features (note that
Google’s implementation of OAuth 2.0 is not entirely standard, so this
OAuth 2.0 authoriser cannot be used outside of Google’s services). It
includes full unit tests too.

New API:
 • GDATA_OAUTH2_REDIRECT_URI_OOB
 • GDATA_OAUTH2_REDIRECT_URI_OOB_AUTO
 • GDataOAuth2Authorizer

https://bugzilla.gnome.org/show_bug.cgi?id=646285
tests: Call setlocale() on startup

So that the correct character encoding is set for all tests.