adding changelog an readme informations on plugin viewer

I've been added an simple modal dialog which displaying the readme (readme.md) and changelog (versions.md) informations for each listed plugin by selecting the plugin name or plugin version link.

![plugin_versions](https://cloud.githubusercontent.com/assets/841998/4658848/037841e4-5505-11e4-89e6-aed01e612860.png)
![plugin_readme](https://cloud.githubusercontent.com/assets/841998/4658851/06e9f156-5505-11e4-9b0a-9c3d8df7d895.png)