add IPython.get_ipython

library entry point for get_ipython

does not rely on injection into globals
Don't rely on `get_ipython` in builtins in library code
don't use deprecated ipapi.get

It's been deprecated for three releases.  Come on guys.
remove reference to nonexistent 0.14 in deprecation warning
move core.ipapi -> core.getipython
remove deprecated `get`
Merge pull request #3286 from minrk/get_ipython

don't use `get_ipython` from builtins in library code

mainly adds IPython.get_ipython() as a library-level handle that does the same thing.