Delay importing sys.getrefcount until needed

This makes it possibly to at least install on PyPy

Addresses: GH-4 ("pypy compatibility")
Merge pull request #9 from msabramo/delay_sys_getrefcount_pypy

Delay importing sys.getrefcount until needed
Change / to // (integer division) in several places

This fixes a bunch of bugs and test failures in Python 3, which uses
"true division" for /