Metadata Bug Fixes

Resolves issue #80

```
$ sl my network public
:...............:...................:
:          Name : Value             :
:...............:...................:
: mac addresses : 06:8a:0d:8c:d8:80 :
:        router : fcr04.dal05       :
:         vlans : 1134              :
:      vlan ids : 236267            :
:...............:...................:
```

```
$ sl my datacenter
dal05
```

```
$ sl my user_data
No user metadata.
$ echo $?
2
```

* Since the REST API gives errors (if you're using json) in json format, I made make_request raise an exception (if it can) based on the json body instead of just HTTP Error codes.
* Fixed datacenter CLI doc block
* Fixed datacenter_id CLI doc block
* Improves user_metadata error when there is no metadata
* Fixes typo/refactor issue that broke `sl my network public/private`