Adding an .editorconfig to Spree

Adding a [.editorconfig](http://editorconfig.org/) will help keep stylistic consistency in the Spree codebase.

While most of us have these rules set up at an IDE level, many Open Source projects - [Ruby](https://github.com/ruby/ruby/blob/trunk/.editorconfig) included - have one defined. So why not Spree?

Developers without the plugin for their favourite editor can download it here - http://editorconfig.org/#download

I have compiled this editorconfig from ones I have used in the past on other projects, however I am open to refining it get a common consensus.

If this PR is accepted should the installation of the plugin be suggested in the syntax section of CONTRIBUTING.md? 
I'm happy now it's 2 space soft tabs.