Add check for free variable assignment

Free variable assignment in coffeescript is just pure evil.
Add a check to warn against that.
For all local variables, use a do block -

do(local_var1 = v1,
   local_var2 = null) =>
  local_var2 = some_intermediate_computation()
  get_result(local_var2)

	modified:   src/coffeelint.coffee