Add Aliyun OSS storage option.

[Aliyun OSS](http://www.aliyun.com/product/oss/) is one of most used storage solutions in China, like S3.

We need store files in local storage host, it will faster that Dropbox, S3...

This pull request add `:aliyun` option for storage.

Usage:

```ruby
store_with Aliyun do |aliyun|
  aliyun.access_key_id = 'my_access_id'
  aliyun.access_key_secret = 'my_access_key'
  aliyun.bucket = 'foo'
  aliyun.path = ''
  aliyun.keep = 2
end
```