Expose free disk space for databases and indexes

Three use cases:
* To easily hook up monitoring tools to a database.
* To enable triggering compaction using an external source based on disk usage.
* To make it make it easier for automatic replication/sharding orchestration mechanisms to make good decisions on how to balance/move data.

The last of these use cases was what triggered me into coming up with this pull request.

Counter arguments against this pull request (that I can come up with):
* Security: One might not want to expose the available disk space to the internet. Do you think I should make this information hidable using an option?
* Dependence on a(nother?) system call when checking status of databases and/or indexes.
* This information can be extracted using mechanisms outside of CouchDB, such as `ssh machine df -h`.

I'd love to hear your input.