Add scheme setting

This adds a "scheme" admin setting and some better instructions on how to set up the plugin. 
This update also makes the filter logic independent of the port, which in theory could change depending on whether you choose HTTP or HTTPS.

Below is a screen shot of the new settings.
![adminsetting](https://cloud.githubusercontent.com/assets/553771/3322050/9696d94e-f741-11e3-9e89-374e6db36456.png)

Do we need a migration to set a default value for this new `_scheme` setting?