Adding support for Amazon S3 rivers

Hello David,

here is a PR for issue #58 : Add a S3 connector.

Everything works well, however I still encounter a bug on the settings page : the AdminRiverServices for FS, Dropbox and Amazon S3 return each an instance of my newly created river ... The get() method does not seem to filter River according the generic type given by the AdminService. I'll tackle this if having any time.

I've choosen the Dropboxrivers branche cause it was my starting point ; but I think you should handle this on a different branch regarding compatibility issues between the 2 plugins when running into embedded mode. Do you have any idea no how to manage to package a scrutmydocs release ? Through different assemblies (standalone vs external ES) ?

Regards,