finally chosed a license: MIT

+ added license text to each public header
some cleanup in ghapi

- removed std::string overloads of some Thing functions
  (don't doing this kind of stuff for now)
* cleanup includes
implement AuthenticatedUser

as specialization of an usual User

see #2
overload authentication via Manager

see #2
implement update of authenticated user

- removed specialized User class
  (not required for that)
* adjusted and tweaked Thing and User
* Manager now uses shared pointers to Things

see #2
update Manager docu

finalizes fixing #2