Prerender

Changed
- preserve phantom arguments when server is restarting
- use default when phantomArguments is empty

Changed
- prevent multiple phantomjs instances from being started with low number of iterations
- try to check to see if phantomjs has actually been disposed. if not, force kill it.

Changed
- added engines to package.json and fixed possible bug in checking options passed in
- prevent weird hangup on error setting a header with a newline
- make sure we catch any errors thrown from phridge and continue
- kill workers (and phantomjs) on SIGTERM

Added plugin to send a header of X-Prerender: 1 with every request

Changed
- Changed `PHANTOM_CLUSTER_NUM_WORKERS` to `PRERENDER_NUM_WORKERS` in server.js
- Changed `PHANTOM_WORKER_ITERATIONS` to `PRERENDER_NUM_ITERATIONS` in server.js
- Switched from `phantomjs-node` bridge to `phridge`
- All Prerender plugins that access PhantomJS need to be rewritten to support new [phridge](https://github.com/peerigon/phridge) integration.
For example, change this:

req.prerender.page.set('onConsoleMessage', function(msg) {
console.log(msg);
});

to this:

req.prerender.page.run(function() {

this.onConsoleMessage = function(msg) {
console.log(msg);
};
});

Please see [phridge](https://github.com/peerigon/phridge) for more info on how to interact with PhantomJS through `phridge`.

Removed
- Removed `PHANTOM_CLUSTER_BASE_PORT` since `phridge` doesn't start a webserve to talk to PhantomJS, so it's no longer needed.
- Removed `PHANTOM_CLUSTER_MESSAGE_TIMEOUT` since `phridge` doesn't start a webserve to talk to PhantomJS, so it's no longer needed.