Replace 'undefined' with 'null' in uninitialized variables

'undefined' is the value of an unknown variable, or rather the lack of a
value. 'null' is the lack of a value in a *known* variable.

See: http://stackoverflow.com/questions/801032/why-is-null-an-object-and-whats-the-difference-compared-to-undefined
Changed healthbar design

Removed text in html, added border
to health bar and added changing
color to the bar depending on the
healthscore