Detailed frontend test reports

Provides this kind of output:

```
$ make frontend
bin/run_selenium_test.sh python -m unittest discover -v test/frontend
+ bin/selenium.sh start
Selenium server started (selenium-server-standalone-2.33.0c.jar, pid=31442)
+ set +x
+ python -m unittest discover -v test/frontend
test_addressbook_index_html (test_frontend_all.FrontEndSuite) ... FAIL
test_chat_index_html (test_frontend_all.FrontEndSuite) ... ok
test_index_html (test_frontend_all.FrontEndSuite) ... ok
test_port_html (test_frontend_all.FrontEndSuite) ... ok
test_sidebar_html (test_frontend_all.FrontEndSuite) ... ok
test_webrtc_index_html (test_frontend_all.FrontEndSuite) ... ok
test_worker_index_html (test_frontend_all.FrontEndSuite) ... ok

======================================================================
FAIL: test_addressbook_index_html (test_frontend_all.FrontEndSuite)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/niko/Sites/talkilla/test/frontend/test_frontend_all.py", line 56, in test_addressbook_index_html
    self.check_page(SERVER_PREFIX + "addressbook/index.html")
  File "/Users/niko/Sites/talkilla/test/frontend/test_frontend_all.py", line 40, in check_page
    raise AssertionError(self.get_failure_details())
AssertionError:
1 failures encountered:
should fail
expected 'plop' to deeply equal 'plip'

----------------------------------------------------------------------
Ran 7 tests in 14.145s

FAILED (failures=1)
+ RESULT=1
+ bin/selenium.sh stop
Selenium server stopped
+ set +x
make: *** [frontend] Error 1
```