Changes to front and back end to handle disk resources (as per #12).

Fairly simple change, I've tested that resource offers don't get accepted if they don't match and amended the existing tests. A few potential questions though:

1. Default is currently set to 0MB - which I assume means any disk resource gets accepted. This could be dangerous if an application starts using disk resources? Is it better to set a low default (say 500MB)?
2. A user's existing applications will not have the znode field added that represents the disk resource - instead the default value will be used. The user will have to destroy and re-add the application for this field to be added to the Zookeeper configuration. Is this acceptable behaviour?