For consistency, enable ESLint `one-var` rule to require multiple variable declarations for initialized variables per scope. Uninitialized variables can still be declared together (preferred) or separately.
http://eslint.org/docs/rules/one-var
Default options can now be accessed by importing `core/core.defaults`. The returned object acts as a singleton and is populated when importing classes that expose their own default values (meaning that importing only `code.defaults` results in an empty object). Also make `Chart.Ticks` and `Chart.Interaction` importable since existing defaults rely on these values.
Add the `defaults._set` method that make easier declaring new defaults by merging given values with existing ones for a specific scope (`global`, `scale`, `bar`, etc).
Properly export helpers and remove dependencies to `Chart.helpers`. Helpers can now be accessed from `src/helpers/index.js` (`var helpers = require('path/to/helpers/index')`, instead of `var helpers = Chart.helpers`).
