{accept,dismiss}_alert
Compatability between poltergeist and selenium

* node[:value] is only valid for selenium
* have_content('...', :css=>'') is no longer valid
* poltergeist and selenium disagree on spacing between elements
fix duplicate example description
Fix some possible race-conditions in backend specs

* adjustments_promotions_spec.rb: wait for first coupon to apply
* properties_spec: allow driver to wait for a second tr
* wait_for_ajax should wait for jquery to be defined
* ensure all requests finish at end of spec
Avoid sleep() using regular capybara mathods
don't console.log
bump poltergeist to 1.5.0
switch to poltergeist for spree_backend

Fixes #4383