Improve comments of the client API.

Mention explicitly which classes are most important, and what they do
for you.
Add a tool and an API for running stat against a blob.

The API client will run stat on every host holding the blob to figure
out its status.
Move blubber_block_directory into the directory server directory.

Having it outside the directory server doesn't make sense (it
is the specific implementation of the server) and clutters the
documentation.
Use the actual bound address for reports.

This avoids having something like port 0 in the block directory simply
because the port was assigned automatically.