Better documentation and support for scripts/migrateblobs.py

[Ross Patterson]

The "ZODB.blob WARNING (PID) The `lawn` blob directory layout is deprecated due to scalability issues on some file systems, please consider migrating to the `bushy` layout." is far too opaque and Googling for ways to resolve this issue doesn't reveal any clear answers. The best we have is ajung's typically brutish response: http://plone.293351.n2.nabble.com/where-does-blob-files-go-td4485744.html#a4489635. That doctest isn't nearly sufficient for documenting the use of this script. Finally, the ZODB3 egg doesn't define a console_scripts entry point for the scripts/migrateblobs.py script.

A console_scripts entry point should be added for the scripts/migrateblobs.py script and documentation on how to create the script and use it should be added to the canonical ZODB documentation:
http://rpatterson.net/blog/migrating-a-blobstorage