Added in AwsAdapert, uploads the sitemap to an s3 using the aws-sdk gem

I am working on a project that uses paperclip and aws-sdk, so I didn't want to install carrierwave. I saw that you had made a rake task that does this, but I thought for ease of use I might as well create an adapter.

Requirements
aws-sdk gem
Requires an config/s3.yml file. Set up the same way paperclip does.

The config is just about the same

```ruby
SitemapGenerator::Sitemap.default_host = "http://www.example.com/"
SitemapGenerator::Sitemap.sitemaps_host = "http://s3.amazonaws.com/bucketnamehere/"
SitemapGenerator::Sitemap.public_path = 'tmp/'
SitemapGenerator::Sitemap.sitemaps_path = 'sitemaps/'
SitemapGenerator::Sitemap.adapter = SitemapGenerator::AwsAdapter.new
SitemapGenerator::Sitemap.create do
  #your set up here
end
```

I hope this helps!