[#1859] Clean pyc files and speed up directory delete operation.


Problem description
===================

paver clean was not removing pyc files, so when switching between python2.7 and 2.8 we had a lot of nasty errors.


Changes description
===================

clean was changed to delete pyc files.

As a side fix, I have also tried to speed up clean operation on windows as it was ridiculous slow. 3min on Windows vs 1 second on Linux. 


How to try and test the changes
===============================

reviewers @alibotean @dumol 

check that clean works on your system and on all OS:
```
paver test_remote all --properties=force_clean=yes
```

Check that changes make sense.

Thanks!