allow persistence.factory in options to be string

[mocheng]

To fix #126

Should we allow factory to be either Function or String? or have a new option field type in persistence?

[mcollina]

Thanks! It should provide a better error reporting in case of an unknown
persistence. Plus, I think we should have them lowercased, just like
Ascoltatori.

[mocheng]

If lowercased is allowed, I suppose it needs a map('redis' => mosca.persistence.Redis, 'mongo'=>mosca.persistence.Mongo, ...). Would that make maintenance more complex? I mean, every time a new persistence storage is added, the code about mapping should updated.

[mcollina]

We usually do not add persistences.

you can do that in a loop in the persistence file, extracting the previous keys with Object.keys() and iterating with a forEach.

[mocheng]

That sounds cool. I'll update the PR.