added APIs: attachments/upload, attachments/download
attachment/download requires nginx's funtions.
* enable http-image-filter-module
* directory {base}/uploaded/images must allow to access directly
* directory {base}/uploaded/etc must allow to access directly
fixed wrong comment
removed unused variable
changed attachments' id from UUID to UUID + extension.
changed uploaded directory structure.
before:
  all files are stored in uploaded/images/ or uploaded/etc/
after:
  all files are stored in uploaded/images/yyyymm/dd/ or uploaded/etc/yyyymm/dd/
added 'gif' to image file types
_id を指定するようにしたところpersisterの戻りに _id が来なくなったので手動で追加
Merge pull request #49 from haushinka2dx/uploadable-attachments

added APIs: attachments/upload, attachments/download