Implement shrink

[nidico]

It'd be very useful if pyqcy implemented shrink functionality similarly to what is implemented in Haskells QuickCheck.

Soenkehahn summarizes:

In case of a failing testcase, QuickCheck's shrink function is used to search for smaller test-data that will still make the testcase fail. It tries to boil the test-data down to the relevant bits that cause the failure. This is immensely useful, since it allows QuickCheck to generate very concise and readable counter-examples to a tested property.

As far as I can see, none of the Python QuickCheck implementations does that.

[pedroteixeira]

https://pypi.python.org/pypi/pyquchk seems do it, not sure if helps