Added support for SSL in URI based connections

Hi there,

Thanks for providing this great plugin.

My project needed to support SSL and this wasn't possible with current version of the Plugin. I have modified SalatPlugin to pass the URI directly through to Salat without decomposing into the constituent configuration properties as was previously being done with MongoSource. To achieve this, I sub-classed MongoSource to provide different implementations based on whether the URI was being used or the sub-configuration elements. This change does not support SSL based configuration using multiple configuration properties, only via the URI MongoSource.

These changes have been tested using Mongo URI's with and without ?ssl=true

Would you take a look at this and merge if you are happy?

Thanks!