Add Poltergeist support.

Conflicts:
	lib/show_me_the_cookies.rb
	show_me_the_cookies.gemspec
updating poltergeist driver for capybara 2
fix overwriting rack-test with poltergeist adapter; duh
Merge pull request #14 from nruth/re-add-poltergeist

Re-add poltergeist, which now has 1.1.0 gem for capy2