Updated markup for getting started
Create extension abstraction entry point

Adds in a new constructor `Extension` that handles the underlying
differences between Chrome and FireFox.  This means that background is
now generated automatically.
Convert contentscript to ES6 module

Now uses the environmnet module as well.
Disable navigation on getting started
Miscellaneous changes, remove files, fix ES6

The ES6 build step was broken, due to:

https://github.com/esnext/es6-module-transpiler/issues/155

I removed unused _asset files.  Also added cross browser support for
message passing from content scripts.
Fix environment imports and integrated settings
Ensure that nested invalid keys don't error
Better styles
Better disabled navigation
Polish up style and markup for getting started
Improved active link detection
Merge pull request #14 from tbranyen/getting-started

Getting started