Fixes to example_full.yml, move imgur_auth to pyfibot/util/

#### example_full.yml
Fix channel password example, was
```yaml
    - pyfibot_test, password
```
which caused bot to send following command to IRC server:
```
JOIN #pyfibot_test, password
``` 
which practically always works but is against the RFC:
```
4.2.1 Join message
       Command: JOIN
    Parameters: <channel>{,<channel>} [<key>{,<key>}]
```
Bot actually expects a list, so now the example is fixed to:
```yaml
    - [pyfibot_test, password]
```

Other changes:

  * add example for channel with non-default prefix
  * add example for twitter bearer token
  * unify the general styling (cases, line width)
  * change some wordings

#### module_imgur.py
move from root to `pyfibot/util/` where it belongs to.