Create image plugin for Bootstrap

This is implements of image plugin for Bootstrap.
I refer to [Bootstrap - images][1].

## How to write image plugin

- as inline plugin
`&image(URL,type,custom-class,alt-text);`

- as convert plugin (give "img-responsive" class automatically)
`#image(URL,type,custom-class,alt-text);`
```
# URL          : url or path or ID for images.
# type         : you can choose "rounded" or "circle" or "thumbnail".
# custom-class : you can set your custom class to write like that "class=custom-class".
# alt-text     : set alt text for the image.
```

## Tasks to Goal

- [x] Add ImagePlugin.php
- [x] Add ImagePluginTest.php
- [x] Add ImagePlugin tests
- [x] Implements ImagePlugin
- [x] Add pull-left & right to convert plugin
- [x] Add tests for convert plugin

[1]: http://getbootstrap.com/css/#images