Updated wp_property_upload_exec module

This updates the wp_property_upload_exec module to use the wordpress mixin and applies rubocop suggestions.

I found a vulnerable version of the plugin on Github (deleted on wordpress.com). To install it, execute the following from your `wp-content/plugins/` folder:
```
git clone https://github.com/luva/WP-Property.git wp-property
chown -R www-data:www-data wp-property/
```
And activate the plugin in the backend.

Output:
![11](https://cloud.githubusercontent.com/assets/105281/3739886/5bd30304-174f-11e4-8020-67d8cdef0594.png)