Sometimes it is useful to transfer a calculated value from the master
to the slave. Instead of having to create a temporary file and then
use FileDownload, you can use one of the string download steps.
StringDownload works just like FileDownload except it
takes a single argument, s, representing the string to download
instead of a mastersrc argument.
JSONStringDownload is similar, except it takes an o
argument, which must be json serializable, and transfers that as a
json-encoded string to the slave.
JSONPropertiesDownload transfers a json-encoded string that
represents a dictionary where properties maps to a dictionary
of build property name to property value; and sourcestamp
represents the build's sourcestamp.