Converge codes for user name restriction

The current implementation accepts only "admin" user. Codes for the
restriction exist in several files.

This change converges codes for the restriction to
app/models/user.rb. This change will be helpful when we support more
user names.
Add translation for processing.
Hide install button while processing.
Show processing status.
Add draper gem to decorate a model.
Refactor logic in view using decorator.
Change the priority of checking status because both installed and not installed plugins could be processing.
Add spec for plugin's decorator.
Change status depending on starting plugin install.
Merge pull request #83 from kou/converge-admin-user-restriction

Converge codes for user name restriction
Merge pull request #85 from hassaku/show_processing_status

Show processing status of plugin install
Display success message when password updated successfully
Merge pull request #86 from Kyuden/success_message_when_password_updated

Display success message when password updated successfully
Organize screenshots in docs directory.
Delete optional div tag in haml
Suppress "already initialized constant KEYS" warning

`Class.new {...}` uses a closure to define a class. Closure keeps
environment that includes local variables and so on. Assignment changes
the environment. So `klass = Class.new { KEYS = [] }` doesn't define
`klass::KEYS`. It defines `Object::KEYS`.

    a = 1
    Class.new do
      a = 2
      X = 3
    end
    p a # => 2
    p X # => 3

We need to use `const_set` to define a constant in `Class.new {...}`.
Merge pull request #88 from Kyuden/delete_div

Delete optional div tag in haml
Merge pull request #89 from kou/suppress-already-initialized-constant-keys-warnings

Suppress "already initialized constant KEYS" warning
Merge pull request #87 from hassaku/organize_screenshots

Organize screenshot images in docs directory.
Cleanly exits from commands.