Move settings from tutor.js to settings.js

There is a global variable called settings with the typing tutor
settings. Move this variable to its own file called settings.js.
Add comments to properties in settings object

Right now there is only one property in the settings object
called TARGET_TEXT_LENGTH. Add a comment to describe this
property.
Use 1 as the minimum length of target text

Check that the value of settings.TARGET_TEXT_LENGTH is positive
before using it. If it is 0 or less, use 1 as the length of the
target text. Update the comment of this property with a
description of the constraints for this property.
Hide tutor code in an anonymous function

Create an object called Tutor that contains a reference to the
init function to start the tutor by making the following
changes.

  1. Hide all code in tutor.js within an anonymous function.
     Expose only the the init function as Tutor.init.
  2. Rename the global object with tutor properties shared
     across all functions from tutor to my. Reorganize this
     global object such that each of its properties is an object
     which in turn contain the actual properties shared across
     the tutor functions. A few examples of such properties are:
     my.html.unitLinks, my.html.subunitLinks, my.current.unitNo,
     my.current.state, my.STATE.READY, my.STATE.RUNNING, etc.
  3. Make the names of some of the HTML elements simpler, e.g.
     target instead of targetText, tips instead of tipsText, etc.