Allowing a HTML/JavaScript interface for Spark to use for logging.

Now if the spark app's HTML includes "packages/tavern/tavern_log.js"
then it can pass "js.context.logMessage" to tavern.getDependencies as
and the log window will appear mirroring the log output.

Known bug to be fixed soon: multiple calls to getDependencies re-adds
the extra logging function so the nth call will show each log message
n times.

R=tapted@chromium.org

Review URL: https://codereview.appspot.com/56400043