Updated follow mode. Got rid of the localhost crap

New strategy for follow mode. Don't check for localhost anymore.

Now, the presenter window is followed, unconditionally. Other windows
are not tracked. Also updates the ping function and adds a disable
parameter so that windows spawned by the presenter are not tracked.

Next steps are to add a key to "secure" the presenter mode and update
function so that the presenter doesn't have to use presenter mode if
they don't want to.
Added a key parameter

If a "presenter key" is set in your `showoff.json` file, then access to
the /presenter endpoint and ability to update the current page counter
will be limited to browser windows with the ?key=<key> parameter set.

To enable follow mode, append ?key=<key> to the URL in your browser.
This will work in either presenter or standard mode. If there is a
"presenter key" set in `showoff.json` then it must match what you
provide.

I am not happy with this solution, so it is likely to change in the near
future, but for now, this is it.
Added leader mode

I think this is how I want it to work. Simply enable leader mode by
pressing the l key. If you have a "presenter key" in the `showoff.json`
file, then you must use `?key=<key>` in the url for leader mode to work.
Merge pull request #44 from binford2k/follow_mode

Follow mode