Don't crash if README_STARTUP is missing

On Windows inside a py2exe'd environment, README_STARTUP does not
get copied into library.zip. Added a warning and a skip if
the file IPython/config/profile/README_STARTUP is missing.
Merge pull request #3212 from jusic/master

fix check_startup_dir inside py2exe

On Windows inside a py2exe'd environment, README_STARTUP does not
get copied into library.zip. Added a warning and a skip if
the file IPython/config/profile/README_STARTUP is missing.

closes #2563