Add oauth_version parameter to OAuth header.

According to [RFC 5949 &sect;3.1][rfc] the `oauth_version` parameter is optional, but the [bitbucket.org API][bitbucket] has been observed to require it.

[rfc]: http://tools.ietf.org/html/rfc5849#section-3.1
[bitbucket]: https://confluence.atlassian.com/display/BITBUCKET/OAuth+on+Bitbucket