Virtuallinks

Initial version, passing all 40 tests in python 2.6, 2.7, 3.2, 3.3, 3.4, 3.5 and pypy.

Main functions implemented:
- `link`, `unlink`, `unlink_all`
- `monitor`, `monitor_typical`, `unmonitor`, `unmonitor_typical`, `unmonitor_all`
- `enable_inspector`, `disable_inspector`