Skip test if pyuv is not available
Actually skip tests: support this on uv.py side
Fix test skipping

Notes:
- Moved pyuv import to __new__ because otherwise __del__ is called.
- Moved raise SkipTest to do_test_tree_watcher because watcher is only created 
  on .watch() call.
Do not install libuv, libuv-dev and pyuv

LibUV is not available in travis