Implementation of Tizen SSO APIs

Implementation of SSO APIs as per widl file https://code.google.com/p/accounts-sso/source/browse/widl/signon.widl?repo=libgsignon-glib

Backend that provides core functionality is gSSO and more information about that
component could be found at https://01.org/gsso
Again, I suggest you read the book - effective STL. I do not think it is right place to explain why `empty()` is always better than `size() == 0`. It took too much time for me to explain the experience I had or chromium ones. Anyway, I'll try my best last time for this PR.

The link you posted here is no problem for me, but think about if we switch to other STL. STL standard require `empty()` as o(1), while `size()` is not. Hopefully, you really understand the cause than the reason you're forced to do that.