Do real parsing of boolean query string parameters

The debug query string parameter accepts anything, even values like "0" and false, as a way to turn on debug mode.

http://crypto.stanford.edu/flashproxy/embed.html?debug=0

Add a get_query_string_param_boolean function and use it to read the debug value. The function should return true when it receives any of these three strings (and only these strings; no case-folding or anything else):

• "true"
• "1"
• "" The function should return false iff it receives either of these strings:
• "false"
• "0" The function should return null in all other cases.

There need to be a couple of simple test cases added to flashproxy-test.js.