Memorise branch

[matsjoyce]

Fixes for #49 and #43.

Latest performance:

Operation	Memorise time	Master time	Slow down
Import dill	0.16855	0.09794	1.721
Dump small python module	0.00242	0.00029	8.213
Dump small python module which imports sys	0.00281	0.00033	8.404
Dump large python module	0.02601	0.00883	2.945
Dump large python module with attr changed	0.02548	0.00871	2.925

I've replaced abc with a large, non builtin module, as all master does with builtin modules is pickle a ref, which make comparison difficult.

[matsjoyce]

This should probably only be activated explicitly (dill.use_diff(on=True)), by people which want this behaviour, as this will help with compatabillity, etc.

[mmckerns]

that's a decent idea to get it into the trunk, at least.

[matsjoyce]

OK, done that. Also renamed to diff.py, because that will make more sense. The current implementation will only add overhead if the use_diff function is called. The only "bad" point is that it introduces another global variable, _use_diff.

[mmckerns]

merged, but disabled in d8d566c, as the diff module is a bit raw. Will post issues as new tickets.