Update Documentation for 3.0.0

[DaniRey]

Following parts of the documentation require an update

• [x] JSON module #67
• [x] Getting started and Download (update the version numbers)
• [x] new chapter describing how to use other browsers #74
• [x] search for references regarding Scala 2.10
• [x] write the release notes, including migration description
• [x] document which env variables or -D parameters are available
• [x] document how configuration changed in general #12
• [x] document how and when we can access response codes and response headers #21
• [x] improve visualization of gauges

[DaniRey]

With https://github.com/unic/ScalaWebTest/commit/146e1dc7fec3f42176b197ce94c4a0dc132b24ae the most important parts are covered.