Add Plugin URI to WordPress Plugin Header

It's nice to have a plugin uri so users to find the plugin.

It also looks silly on the WooCommerce System Status page without one. :) 

![woocommerce_status__wc_2_1_x__wordpress](https://f.cloud.github.com/assets/1065372/2208506/9f6f722a-9980-11e3-8089-a42511fd136b.png)