Auto-expiration of audit messages

Resolves #116 

Using the current implementation of the expiration bundle, one must specify expiration date explicitly for each individual message. See https://github.com/ayende/ravendb/blob/master/Raven.Database/Bundles/Expiration/ExpiredDocumentsCleaner.cs#L47

While the deletion process runs once every 24hr (see previous commit), in our scenario it probably makes more sense to define the expiration index differently - to use the configured time span for deletion when deciding which document to delete. We can probably also reuse the standard messages index.

For now I enabled expiration using the built-in bundle. I think we want to roll our own bundle by mostly copy-pasting so expiration is reusing the main MessagesView index, and decides to delete based on the configured timespan compared to the message timestamp.

Thoughts?

/cc @johnsimons @andreasohlund