Plugin integration

* plugin usage is now:
Configuration:
```coffeescript
#./server/config.coffee
plugins: [
    'an-americano-plugin'
]
```
Usage:
```coffeescript
americano = require 'americano'
americano.pluginProperty()
```
There is no detection of eventual collision, maybe I can add a warning message if a collision is detected (would only require to rewrite the ```extends``` function, no big deal) ?

* we can specify absolute paths for plugins (to ease plugin development/debugging)
* in the "configure" method of plugins, you can use "this" as americano instance

Before merging, I think we could discuss the plugin configuration so I can add it to the PR.
To keep things simple this is what I propose:
```coffeescript
plugins: [
    'plugin-name': 
        'configKey1': 'config value 1'
        'configKey2': 'config value 2'
    ]
```
Then we could change the "configure" plugin methods by adding the options:
```
coffeescript
module.exports.configure = (root, app, options, callback) ->
```

Tell me what you think and I will add the proper changes.