Improve network state callback

[happybeing]

When calling safe.initialiseApp() a callback can be provided to report changes in network state. These appear to be returned as strings (I see 'Connected' on successful init for example) and are presumably set here:

https://github.com/maidsafe/safe_app_nodejs/blob/83a5bc47575270723c2fa748cde104d0ec770250/src/app.js#L479-L497

Issues:

• the 'state' type/values are not documented in the API docs for initialiseApp()
• network state is set using 'magic' values in the code above (and not part of the internal consts - see above)
• network state values are not available to the app for comparison and should be exposed on safe.CONSTANTS (actually I think a lot is probably missing from here)
• reporting network state using numeric codes rather than strings may be preferable, e.g. it may simplify localisation in apps. In general I don't think reporting state using strings is a good idea, but maybe that's a personal preference.