Add content source list admin command.

Adds:
- The *content sources* section to the admin command structure.
- The *list* command.
- Recipe to user guide.
I have different output.  I don't see a Paths attribute.  It's called URL.  Here is my/etc /pulp/content/sources/conf.d/local 
```
[content-world]
enabled: 1
priority: 0
expires: 3d
name: Content World
type: yum
base_url: http://contentworld/F-20/GOLD/Fedora/
url: /x86_64/os/
max_concurrent: 10
max_speed: 1000

[content-world2]
enabled: 1
priority: 0
expires: 3d
name: Content World
type: yum
base_url: http://contentworld/F-20/GOLD/Fedora/
url: /x86_64/os/
max_concurrent: 10
max_speed: 1000
```

and my output for the command ...
```
[dkliban@localhost pulp]$ pulp-admin content sources list
+----------------------------------------------------------------------+
                            Content Sources
+----------------------------------------------------------------------+

Base URL:       http://contentworld/F-20/GOLD/Fedora/
Enabled:        1
Expires:        3d
Max Concurrent: 10
Max Speed:      1000
Name:           Content World
Priority:       0
Source Id:      content-world2
SSL Validation: true
Type:           yum
URL:            /x86_64/os/

Base URL:       http://contentworld/F-20/GOLD/Fedora/
Enabled:        1
Expires:        3d
Max Concurrent: 10
Max Speed:      1000
Name:           Content World
Priority:       0
Source Id:      content-world
SSL Validation: true
Type:           yum
URL:            /x86_64/os/
```