Common object storage frontend

Common object storage frontend that supports dictionary-style access, object
serialization and compression, and multiple storage and caching backends.

Currently supported storage backends are:

* Amazon S3 Web Service
* Berkeley Source Database
* Filesystem
* Firebird
* FTP
* DBM
* Durus
* Memory
* Microsoft SQL Server
* MySQL
* Oracle
* PostgreSQL
* SQLite
* Subversion
* Zope Object Database (ZODB)

Currently supported caching backends are:

* Filesystem
* Firebird
* memcached
* Memory
* Microsoft SQL Server
* MySQL
* Oracle
* PostgreSQL
* SQLite