Add comments to object return statements

The objects Tutor and Util in tutor.js and units.js are defined
with the revealing module pattern. The return statements in the
anonmyous function that returns the object with the revealed
functions have no comments. Add comments to these return
statements.
Remove semicolons from anonymous function calls

In this project, by convention, semicolons are not used at the
end of JavaScript statements. However, the anonymous function
calls that define the Tutor and Util objects are terminated with
semicolons. Remove these semicolons for the sake of consistency
in order to conform to the coding convention used in this
project.
Add 5-6 split units for number keys (Units 16-20)

In most keyboards, the '6' key is located closer to the 'F' key.
As a result, the '6' key is more accessible to the left hand
than the right hand. Therefore, many people prefer the
traditional style with 6-7 split where '6' is pressed with the
left hand and '7' is pressed with the right hand. However, there
are also many people who prefer the nontraditional style with
5-6 split. Therefore, add support for 5-6 split typing for users
who prefer this by providing the following features:

  1. Add a link below the input box called '6-7 split' or
     '5-6 split' only for units which introduce the number keys
     to the user, i.e. units 16-20.
  2. When a user clicks on one of these links, the corresponding
     unit is loaded.
  3. Update README.html with information about the alternate
     units.
  4. Fix README.md so that it mentions units 16-20 as the units
     that introduce the number keys.