DFP to GAM, DFW to SCAMP

[benlk]

Changes

• Rebrands the plugin to Super Cool Ad Manager Plugin
• Replaces references to DFP with GAM

Why

• Because of https://www.blog.google/products/admanager/introducing-google-ad-manager/

For #69

Testing/Questions

Questions that need to be answered before merging:

• [ ] where is the screenshot in https://wordpress.org/plugins/doubleclick-for-wp/ coming from and how do we update it?

[benlk]

The screenshot is coming from something in SVN, and needs to be deleted through there. Adding that to the release tasks now, and merging this.