Savely close test resources

This resolves most of the "permission denied" errors identified in #71.

However, there is still one error occasionally popping up in the FS test. Maybe you can have a closer look?

    ======================================================================
    ERROR: test_write_sync (__main__.FSTestWrite)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "test_fs.py", line 576, in setUp
        self.fd = pyuv.fs.open(self.loop, TEST_FILE, os.O_WRONLY|os.O_CREAT|os.O_TRU
    NC, stat.S_IWRITE|stat.S_IREAD)
    pyuv.error.FSError: (50, 'operation not permitted')