Add the opportunity of proxy handling to the oauth2 module

I adjusted the oauth2 module to pass proxy settings as an optional parameter to the constructor.
If proxy settings are available, the options object for the _executeRequest method will be extended with them. 
A sample proxy-object looks like this : 
```
 proxy : {
    host : 'port',
    port : 'host',
    protocol : 'http' // http / https
 }
```