added shaded jar build and instructions for use in deploy:deploy-file

[alexnauda]

I'd like to use this S3 Wagon provider for deploy:deploy-file operations and possibly other invocation types that are outside of the context of a project. I thought this might be helpful to other users of aws-maven, so here is a way you might share that (leaving the relevant pom.xml section commented out but including instructions on the README to help users who are less familiar with custom wagon providers).

This is similar to #29 though it doesn't solve that issue.

[alexnauda]

@blasd modified this to use a profile instead of uncommenting a block in the pom. Isn't it nice?