Better documented behavoir for override, drop overwrite

[henryiii]

Fixes #245. Please comment on whether this is a good plan, or provide suggestions.

Same behavior as current Plumbum if no arguments supplied.

[coveralls]

Coverage decreased (-0.02%) to 82.498% when pulling eeb07bb on henryiii-override into 456d2c3 on master.

[AndydeCleyre]

Here there is a mismatch in base.py:

def copy(self, dst, override=False):

Will silently fail to copy if override is None (the default).