Use proper file system change monitoring

[objarni]

Right now, pytddmon uses polling to check for file system changes - scanning the directory and subdirectories it's running in checking if any .py file has changes since last time the tests were run.

A much less resource intensive method would be to use some kind of lower-level file system event API. However, there does not seem to be a built in API for this in Python, so reliance on some third party library would be necessary, which goes against pytddmons idea of being as independent as possible (copy paste distribution).

If pytddmon is made available for pip installation, that would no longer be a problem, or at least less of a problem, since then pip would install the depending library first.

Research needed.