New Strategy: Spy which can imitate the interface of an instance of a given class without instantiation

[acook]

Use case:

• To mock objects which have a high instantiation cost or utilize I/O
• Database and file objects

Criteria:

• Spy accepts a class as target
• Class is never instantiated
• Spy responds to methods that an instance of the class would
• Spy raises for methods that an instance of the class doesn't respond to

Extracted from #6