Add a plugin for determining disk usage.

This plugin utilises the work in gh-2919 to provide a summary of disk usage by object type:
```
bin/omero usage Image:1,2,3  --report
Using session b011dfc4-2d21-4898-b72b-0a366891414a (user-0@localhost:4064). Idle timeout: 10.0 min. Current group: pg-0
Total disk usage: 153629 bytes
 component    | size  
--------------+-------
 Job          | 59124 
 Annotation   | 8     
 FilesetEntry | 90432 
 Thumbnail    | 4065  
(4 rows)
```
This is very much work in progress in both the name, `usage` is not that good, the object arguments and the output styles, more for discussion than review at this stage.

/cc @mtbc @joshmoore @sbesson

--depends-on #2919
The detailed breakdown is printed in addition to the total. Again, if anything is going to use this automatically do we want to be able to have the just the breakdown without the total, ie one, the other or both.