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 (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 

The link is displayed in the following templates:
- shared/order_details
- admin/shipments/index
- shipment_mailer/shipped_email

We may want to add the tracking to the order listing on the account page too, since many users are going into the account page to check on the status of their orders. 

![Edit Shipping Method](https://f.cloud.github.com/assets/4437/213653/866e01ae-839e-11e2-8199-07067e333e79.png)


![Empty Tracking Placeholder Text](https://f.cloud.github.com/assets/4437/213654/8e7ce036-839e-11e2-8212-e94d4a160c79.png)


![Order Screen Tracking Link](https://f.cloud.github.com/assets/4437/213656/cf2ddc52-839e-11e2-922a-12bfc2aa8fd6.png)