Added support for multiple selenium tests.

- fixed problem with selenium tests in multiple files
- added test to check for reading a single mail
- fixed shutdown error in httpd.py (socket not closed)
Added gui test for mark read/unread.

- Added some test convenience methods
Fix for race condition while checking for class absence or presence.
Merge pull request #551 from lieblingswelt/lj-guitest-unreadbutton

Added gui test for mark read/unread.