coros: reimplement Semaphore similarly to how Event is implemented; add rawlink() and wait() methods

- add DummySemaphore class

http://code.google.com/p/gevent/issues/detail?id=24

- it's no longer used by gevent itself

- add environ argument which can be used to override WSGI's environ passed to the handlers
- set log to sys.stderr if None is passed
- set wsgi.errors to stderr if it was not set otherwise; do not reference sys.stderr from base_env class variable