Added replacement stub for cstdlib atexit() funciton.

This is an empty stub to simply allow use of complex types with a
non global static lifetime. For more complex handling the function
'atexit' can be redefined in user code.

For more information see:

https://github.com/arduino/Arduino/pull/2229
https://github.com/arduino/Arduino/issues/1919
Merge branch 'master' into ide-1.5.x