Adds tracking_url field to shipping methods

This feature adds the ability for tracking numbers to be clickable throughout the system. 

Without it, the customer needs to locate the tracking website themselves and copy and paste the tracking number into it (except gmail users).  It also effects admins who are trying to investigate the history of a shipment, it would be helpful if they could just click on the tracking and open up the tracking site.

A new column `tracking_url` was added to `ShippingMethod` model. The `tracking_url`  stores an example tracking link with the token `:tracking` in place of the actual number. e.g. **http://quickship.com/tracking?n=:tracking**. The `:tracking` part is interpolated with the actual tracking number at runtime. 

This new field is totally optional, when left empty everything reverts to unlinked tracking numbers 

P.S. we might want to add a tracking link to the orders/show screen as well