#261: Import of Evented from Utils set instead of global scope

[satanTime]

When building library through webpack there're 2 global dependencies of tether.js on TetherBase and Evented, however Evented is part of TetherBase.Utils and we can import it from there.

Without fix import code looks like:

global.TetherBase = require('exports-loader?TetherBase!./node_modules/tether/src/js/utils.js');
Object.assign(global, global.TetherBase.Utils);
global.Tether = require('exports-loader?Tether!./node_modules/tether/src/js/tether.js');

With fix

global.TetherBase = require('exports-loader?TetherBase!./node_modules/tether/src/js/utils.js');
global.Tether = require('exports-loader?Tether!./node_modules/tether/src/js/tether.js');

[RobbieTheWagner]

We're now importing Evented as a separate class, so I believe this is no longer an issue.