Uploadable

## Overview

Enabled to upload everything, download everyting, preview images on timeline, view full images.

## You can test on staging environment

https://atmos.interprism.co.jp/atmos-web-stg/
id/pass -> bob/BOB

## Screenshots

### Attachment files on timeline
![attachment-timeline](https://f.cloud.github.com/assets/2914192/2176081/47707096-95cb-11e3-80a6-ea3d41cb06e3.png)

### View Fullsize Image
![attachment-image](https://f.cloud.github.com/assets/2914192/2176082/4d59fa4a-95cb-11e3-84d7-40fe2126a6e2.png)

### Download non-image file by clicking icons on timeline
![attachment-download](https://f.cloud.github.com/assets/2914192/2176084/5146b1e8-95cb-11e3-9f05-78679e9c3dcf.png)

### Message Sender Panel with Upload Zone
![attachment-sender-initial](https://f.cloud.github.com/assets/2914192/2176086/53a5e4a4-95cb-11e3-9042-7f47b4854e97.png)

### Drag and Dropped image file
![attachment-sender-image](https://f.cloud.github.com/assets/2914192/2176087/5645bc98-95cb-11e3-9384-ab5450ed5bd2.png)

### Drag and Dropped non-image file
![attachment-sender-zip](https://f.cloud.github.com/assets/2914192/2176089/58c657c0-95cb-11e3-81b7-c417b750e532.png)

## Limitations

1. Max file size is 100MB
2. Upload was executed **immediately** when you drop file to Message Panel's zone
3. Also, upload can not be canceled
4. Uploaded files are **PUBLIC**.  (server API's limitation)
5. Uploaded files can not be removed by user (server API's limitation)

## Remarks

* fixed #14 by this PR