Small typo fix (Client instead of client)
First param is erased if function

In core.api, if the params parameter is function, it is erased. getToken of client uses first parameter as parameter to core.api and gets always erased if oauth2.client.getToken() is called with first param as callback.
Edited readme to reflect this change
Merge pull request #30 from PuKoren/patch-1

Client flow example fix