Pull request to make the cukes work (especially in SQLite)

All tests pass (in SQLite, with Selenium and Webkit). (Also, they pass when set up for PostgreSQL—just for myself.)

Added optional environment variable CAPYBARA_JAVASCRIPT_DRIVER (defaulting to 'selenium') with meaningful settings 'selenium', 'webkit', 'webkit-debug' and 'poltergeist'. (I didn't try Poltergeist.)

Also added optional environment variable FIREFOX_BINARY_PATH.

@tmock12, does this work for you? or would you suggest any changes?