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

Google have changed their downloads so that older versions are no longer accessible without logging in and no longer have long-lasting, fixed URLs. This causes our Buildout scripts to break all the time.

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.

Related issue on the App Engine issue tracker - https://code.google.com/p/googleappengine/issues/detail?id=10840.

Also, there are a few documentation fixes here and there.