Xengineer/remove user from organization

# brief description
---
enables admin user to remove user from an organization.

# Screenshot
---

- members list and remove button
![screenshot-designax - google chrome-4](https://cloud.githubusercontent.com/assets/1254996/3372478/61baa39a-fbab-11e3-8453-345914b58080.png)

- remove pressed and the confirm dialog shows up
![screenshot-designax - google chrome-1](https://cloud.githubusercontent.com/assets/1254996/3372492/7f90f1a8-fbab-11e3-82a7-924e8006425d.png)

- can't remove yourself
![screenshot-designax - google chrome-2](https://cloud.githubusercontent.com/assets/1254996/3372498/8d394c6a-fbab-11e3-9b23-9fa3a4bc2811.png)

- removed successfully
![screenshot-designax - google chrome-3](https://cloud.githubusercontent.com/assets/1254996/3372504/9a7d958e-fbab-11e3-9788-aa560292d2ae.png)

- if current user is not admin, don't show remove user button
![screenshot-designax - google chrome-5](https://cloud.githubusercontent.com/assets/1254996/3372633/9a422476-fbac-11e3-99f4-8f0efd0f3452.png)

# 今後
---
- メンバー一覧で誰が権限持ってるかわかるようにする
- 権限の付与・剥奪
```ruby
delete :destroy, organization_id: organization.id, id: target_user.id
```

にして、操作者（= signin user）は固定にしておいて、contextによって
```ruby
let(:target_user) { user } # 自身を除名しようとした場合
let(:target_user) { create :user } 
```

みたいにtarget_userを入れ替えていくのが自然かも。