allow scenario with multiple browsers

[bhecquet]

Selenium scenario always play with one browser only, but in some case, an other browser may be necessary to do some parallel operations:

• Do something with chrome
• start IE and do other things on an other application that do not support chrome
• go back to chrome to finish scenario

We should be able to:

• create an other browser, given a session name
• switch to the newly created browser
• go back to an other browser (session name: "main"
• close all managed browser at test end