Changes in runner

Overview of changes: 

* Servers are represented as an intance of ``provy.core.server.ProvyServer``, this adds the benefit that you actually know what can be defined in server. This class also is responsible for the parsing from server info from provyfile. 
* Server (represented as object) is now passed to the context. This is required for the upcoming rsync functionality, as rsync launches it's own ssh connection to the server, and so needs to know credentials. 
* You can now query name of the server, so if you had: 
   ```
   servers : {
      'foo' : {'address': 'bar', ...}
  }  
  ```
  now ``Server.name`` will be ``foo``. I'll be needing this feature for some new things I'll use provy for.