added editorconfig

[sbmaxx]

Just added .editorconfig for convenience ;)

By default a lot of people has 4 spaces indentation, but in polymer — 2 spaces. Editorconfig can help in this difference.

[addyosmani]

Please note that at present, it doesn't appear like any of the other Polymer repos include an editorconfig file by default: https://github.com/Polymer/CustomElements and this may be rejected as team preference.

[sbmaxx]

Including editorconfig in polymer-gestures will not break anything, but may be a signal to other projects begin to use it to make contributing easier. Also, polymer team can think about using JSCS for common codestyle.