Initial setup & basic styles for prototype

#### What's the plan?
To initialize and configure new Middleman project. To add basic styles for FMD.

#### What's this PR do?
It adds the basic structure for a Middleman project, including config.rb, source/, Gemfile, etc. It adds the basic Sass partials structure in source/css/ and basic styles for the prototype.

#### How should this be manually tested?
Get the project cloned down, and then start up the local server by running:
  `bundle exec middleman` 

#### What are the relevant tickets?
This Sprint task: https://basecamp.com/1866004/projects/1175549/todos/122693210
There's a Basecamp project for the FMD site overall here: https://basecamp.com/1866004/projects/6017903/

#### Next steps?
Continue iterating on the FMD site according to art/content as these come in.

#### Smells?
None that I know of here.

#### TODOs:
 - [x] Clean up config.rb
 - [x] Add basic styles for theme sections; add basic styles for content types of images/videos/text
 - [x] Add in basic settings and grid elements
 - [x] Add in Newsapps small footer as a start, as discussed in FMD meeting

#### Has the relevant documentation/wiki been updated if necessary?
README has the basics of getting Middleman up and running; will continue to add relevant project info here.

#### Technical debt note
Adds code. Adds a new (to Tech, but not Texas Tribune in general) static site tool to the Tech arsenal, leading to more of a learning curve for anyone jumping onto or continuing this project if they're not familiar already.