added credentials providers support

close #20 
This code add support for aws credentials resolver. I tested it on https://github.com/ohnosequences/sbt-s3-resolver/tree/credential-providers/src/test/resources/test-a and it workr.

It seams that it will break nice-sbt-setting, because I changed constructor arguments for S3Resolver, but it seams that recent changes of sbt-s3-resolver anyway broke it, so it is not critical.