add new site-id and custom-url options
fix site-id option in README
add basis of site-id and custom-url support
add specs for site-id or custom-url error
extract environment creation to its own method
add spec for flow when using custom-url + site-id
let site-id be usable alone without custom-url
update README with additional info on site-id
add URL validation
add API error handling
change the Ruby version tested by Travis
Merge pull request #9 from rainforestapp/feature/add-ad-hoc-environments

add new site-id and custom-url options