Create a global object to contain tutor properties

The code in tutor.js contains a few global variables. This makes
finding the declaration of each global variable in use
difficult. Create a global object called 'tutor' to solve this
problem. Turn all the existing global variables into properties
of this global object. Thus, wherever a property of 'tutor'
object is used, the property would be prefixed with 'tutor.'
thereby making it clear that the property belongs to the global
'tutor' object.
Replace "instructions" with "tips" for tips pane

The pane that displays how the user should place his fingers to
type the target text contains three HTML elements:
  1. instructionsPane
  2. instructionsLink
  3. instructionsText

Since units.js uses keys called 'tips' to contain the tips to be
displayed to the user, rename the above HTML elements,
respectively, to the following:
  1. tipsPane
  2. tipsLink
  3. tipsText