Memorise branch

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.