Plugin paths broken

In `pelicanconf.py`, this works:

``` python
 PLUGIN_PATH = 'src/pelican-plugins'
```

this doesn't:

``` python
PLUGIN_PATHS = ['src/pelican-plugins']
```

Specifically it crashes with something like `ERROR: Couldn't find <plugin_name>`. The doc examples usually use plugin paths so not sure what's causing this. @dimazest any ideas?
