Pract 4 - Start of Prac 4

Beginning of practical 4 with a new CSS sheet.
Prac 4 - Set em size

Set up the basic structure for the body so that em can be used as a
unit.
Prac 4 - Set default font size for the page

Default font size is set to 1.8 em = 18 px = 14pt font.  Set static
page width.
Prac 4 - Set heading sizes

Set the heading sizes to be increasingly small.
Prac 4 - Set introductory text to be black

Set introductory text element with id intro to black. (updated
complete.css as it was looking for a class not an id)
Prac 4 - Header and nav styling

Added styling for the header element as well as setting a horizontal
navigation bar with inline elements.