Feature/gemify - enable using crabgrass as a gem

Why would we need a crabgrass_core gem?

First of all I want to keep the site specific mods outside of the core codebase. But still they need to be testable. In order for that i need an easy way to setup a dummy cg app that includes the mod in question. Easiest way for that is setting up a blank rails app with a crabgrass_core gem added.

Then the mod could be added as a Gem to a site specific crabgrass_core branch. But I would also like to be able to set a site specific app up like this:
```
Gemfile:
  gem 'crabgrass_core'
  gem 'crabgrass_moderation'
  gem 'crabgrass_superadmin'
  ...
```
This would be in a separate repository. This repository would contain all the site specific changes then.