encode message queue as JSON

If JS included the separator as part of its data, the Obj-C side would
interpret it incorrectly:

WebViewJavascriptBridge.send('Including __WVJB_MESSAGE_SEPERATOR__ in
my message is broken.');

Before:
WVJB receivd: {"data":"Including
WVJB receivd:  in my message is broken."}

After:
WVJB receivd: {"data":"Including __WVJB_MESSAGE_SEPERATOR__ in my
message is broken."}