input/raw_input Python 2 compatibility

In Python 3 `raw_input()` has been renamed to `input()`. This means that when gaffer is used in Python 2, strings have to be entered with quotes. To correct this behaviour, I have aliased `input = raw_input`

I've corrected this in each file that uses `input`. Alternatively this could be put util.py, as suggested in benoitc/gaffer#32.