added the concept of browser dependency, added xvfb as a new dependency, and refactored JUnitSession to start and stop all the dependencies. 
AsyncBrowserSession is now a dependency like any other
- Added headless chrome and firefox. 
- Deleted SharedExternalProcess. AsyncProcess is sufficient
- Made a few changes to error handling logging in JUnitSession
- Applied some temporary fixes to a few problems with Xvfb
Renamed a few classes and resources to better reflect their behavior
Headless versions of firefox and chrome now kill the process they started after finishing the tests. This avoids many instances (or many tabs) of the browsers running invisibly at the same time.
Merged AsyncBrowserSession into Browser. They were to tightly coupled to remain separated.
Forgot a file in the previous commit
add rhino browser in the test helper as a LongPollingBrowser (doesn't seem to be working...)

Conflicts:
	examples/src/test/resources/stjs-test.properties
	test-helper/src/main/java/org/stjs/testing/driver/DriverConfiguration.java
rhino can now happily run next to the other browsers.
Merge remote-tracking branch 'st-js/master' (some browsers are broken)

Conflicts:
	examples/src/test/resources/stjs-test.properties
	test-helper/src/main/java/org/stjs/testing/driver/DriverConfiguration.java
	test-helper/src/main/java/org/stjs/testing/driver/JUnitSession.java
	test-helper/src/main/java/org/stjs/testing/driver/browser/AbstractBrowser.java
	test-helper/src/main/java/org/stjs/testing/driver/browser/RhinoBrowser.java
	test-helper/src/main/resources/start-nopoll.html
Rhino can now run happily next to the other browsers, using the same starting HTML