Add mirrormanager2.lib.pid to manage PID files
Add mirrormanager2.lib.sync to manage sync
Improve the internal API with methods required for the crawler
Replace the LargeBinary fields by PickleType fields

This allow us to directly load the content of the field and thus access
it
Fix the backref from dirs to directories
Update the crawler_perhost.py file that crawls a specified host
Add the crawler script updated for 2.0