Fs cli plugin

In preparation for possibly allowing migrating archived files, unsymlinking files, and similar, the `fs.py` plugin provides some visibility into the contents of the `ManagedRepository` directory. Two commands are currently available:
 * `bin/omero fs repos` which simply lists the available repositories, and
 * `bin/omero fs sets`.
Basic out put from these are of the form:
```
 # | Id | UUID                                 | Type    | Path                               
---+----+--------------------------------------+---------+------------------------------------
 0 | 1  | c8db8f5c-b6ba-472f-b0d5-3dac4f31be66 | Public  | /home/data//ome2                   
 1 | 2  | ScriptRepo                           | Script  | /opt/ome2/dist/lib//scripts        
 2 | 3  | c4df3508-cffe-4bf2-a1c8-b3c1395f737f | Managed | /home/data/ome2//ManagedRepository 
(3 rows)
```
and
```
bin/omero fs sets --limit=3
 # | Id  | Prefix                              | Images | Files | Transfer 
---+-----+-------------------------------------+--------+-------+----------
 0 | 701 | root_0/2014-05/01/21-26-06.464/     | 1      | 2     |          
 1 | 651 | 2365b_1756/2014-04/30/20-51-43.057/ | 1      | 1     |          
 2 | 602 | 2365b_1756/2014-04/30/20-49-24.814/ | 1      | 1     |          
(3 rows)
```

The only available option for `repos` is `--managed` to print out just the row with the managed repo. `sets` provides more options: