Fetch the latest SDK version if 'url' is not specified. Documentation fixes.

This pull request allows us to skip the 'url' option, in which case the recipe will simply fetch the latest version of the SDK and install that. Users who do specify a 'url' explicitly are unaffected.

This PR also checks that the discovered SDK has actually been made public by making a HEAD request to the SDK download URL.

Updated PR fixes problems with #12.