Update to allow activity() to update some system variable values

[imnotbob]

HE change to see some system variables in piston edit screen and update their values: $args, $http, $ifttt.

[idpaterson]

Thanks, this looks good but needs a few fixes:

• Throw in a var here: for (var varName in systemVars) { to define it within the scope and avoid implicitly using window.varName
• Looks like varValD is meant to be used a few lines later as varValueD
• {t: varType, v: varValue d: true}; needs a comma before the d:
• Rebase pull request against the dev branch

[imnotbob]

thx, new request to dev with these fixes. Did your first fix for existing global code