Make interfaces simpler

Clean up code
Separate exercises into separate file
Simpler function names
Add variables exercise

The birds variable can be set to determine number of birds
Simplified some exercises
Added 'Things you've used' to the bottom of the page
Update docco
Fix JSLint errors and remove cruft
Remove jargon
Merge pull request #3 from boxuk/cleanup

Cleanup - lower barrier to entry, more gradual difficulty