Add can_calculate method

Returns true if the calculator has enough information to calculate a
result. Add a different partial for if there's not enough information to
show a result.
Lots of style changes and design tidy up

Thanks @tombye
Change button types
Add buttons in
Make big buttons bold
remove files we dont need any more
Fix CSS the specs are looking for
Calculators -> Quick Answer
Persist if we're showing the annual income forms or not
Move lib class to be a model
Add some content from Natalie in
Add all the content for 2012-13 return
Merge pull request #3 from alphagov/add_child_benefit_calc

Add child benefit calc