Move all plugin files to the root of the repository

How would you feel about this kind of change to the repository structure?

This would make it possible to:

1. Install the plugin to a server simply by cloning it directly into the ```mod/``` directory:
 - ```git clone https://github.com/iionly/Elgg_1.9_poll.git```
2. Update it later simply by doing a pull/checkout directly into the same directory:
 - ```git pull```