Fixes waiting for Ajax in Cucumber:

* old implementation did not actually wait,
  but just checked once whether jQuery or Prototype provided
  any signs of ajaxy activity. It did not actually do anything
  with this information.
* the new implementation waits (busily!) 'til PT and jQuery
  have no signs of ajax anymore. If that takes to long,
  it'll raise Timeout::Error.
Merge pull request #145 from opf/fix/rails3-cucumber-wait-for-ajax

merged anyhow. Maybe we want to add a sleep later but it is definitly not necessary.