GUID.js: set a GUID cookie for unique users throughout a web application session

This plugin is intended to be used in the context of a web application where you want
to trace the path of a user through the page and how bandwidth and performance of the
application inhibits the user from his usual workload in an application.

The technique used in this case is checking wether or not a UUID has been set for the
sesssion as a cookie. It is intended to be used as searchable entity in the later
beacon analysis phase.

Configuration for this plugin may look like this:

```javascript
BOOMR.init({
	GUID: {
	      cookieName: "myCookieName"
	}
});
```

You may use the cookie name as the unique key to search for in your beacon data.
Add GUID documentation
Fix Typo in reporting when Cookie has been found
If setting a cookie fails try to set it again in before_beacon
Properly check for falsiness when testing for existing cookies
Merge pull request #18 from andreas-marschke/feature/guid

GUID.js: set a GUID cookie for unique users throughout a web application session