Enhancements to definitions

Updated the code for the definition tabs to change background and text
color to show which tab is showing the definition. I also added the
jqueryUI items to the project.
Merge remote-tracking branch 'origin/master' into tnichols
Conditionals and Return Home

Added shell for conditionals lesson. I pulled the initial content of
#dynamic in homepage.html into its own file, main.html. I added a Return
Home button that loads main.html, giving a way to always be able to
return to the initial homepage contents.