add ability to exclude some files from deployment

[moredip]

For example, I don't want to deploy test code. Would be nice to have a rake task like:

Microstatic::Rake.s3_deploy_task(:deploy) do |task|
  task.source_dir = File.expand_path("../public",__FILE__)
  task.bucket_name = "foo"

  task.exclude "bower.json"
  task.exclude "spec/*_spec.js"
end

for bonus points:

• support for regexs
• support for passing an array to exclude
• support for passing multiple args to exclude
• support for a dry-run command which just lists what would be deployed (probably worth it's own issue)