Format source based on eslint xo shared configuration

[dinoboff]

Since the next branch modifies most of the source code, we might as well reformat it now and ensure consistent style in the future.

The process:

• convert jshint config to eslint (and fix jshint config to some extend).
• format source code.
• test formatting in CI.
• add command to fix formatting issue.
• add command to report formatting issues with html reporter.
• add contributing notes.

XO overrides:

• set source mode to script instead of module.
• indent with space.
• allow non strict comparison to null.
• allow use before define for constructors.
• allow one line brace style.
• one var for uninitialised vars.
• no space before function paren.

XO Extensions:

• Only use features supported by node +4.3.
• no var (use const or let).
• no confusing arrow function body.
• use bracket for arrow function body as needed.
• new line after variables declaration.
• one line around ‘use strict’.
• require object shorthand.
• prefer arrow callback.
• prefer const.