This patch adds a __str__() method to the ConnectionManager class,
which allows for a nice readible table to be obtained that shows
the current state of the connection pool.  This can be very useful
for troubleshooting or monitoring issues related to connection
pooling.  The table will contain a row for each connection within
the pool, with columns showing the connection slot, connectivity
status, activity status, URI, connection lifetime, and bind DN.
The header row will also indicate the pool size and maximum
connection lifetime setting.

Note that this adds a dependency on the prettytable module.  This
new dependency seems worth it for the nice readible table format
it produces.

Change-Id: If0abfef405d05ecd499bdf6201ff465bd845957b