.editorconfig file for consistency in contributions

EditorConfig defines and maintains consistent coding styles between
different editors and IDEs: http://EditorConfig.org/
4-spaces indent for JavaScript-like files

4-spaces is the most standard indent for JavaScript-like files and this
is what I've set in the .editorconfig file.
Merge pull request #4 from madarche/master

.editorconfig file for consistency in contributions