Moved twisted/reactor.py to platform/twistedreactor.py.

Moved twisted/reactor.py to platform/twistedreactor.py.

twistedreactor.py:
  - cleaned up logging
  - don't try to crash if stop has been called (uses _running to check it)
  - don't add reader and writers if they've already been added
  - use a NullContext when calling add_handler
  - call IOLoop.close() to properly close file descriptors
  - implement run() and instruct the reactor to not install signal handlers.

import_test.py:
  - import twistedreactor too

twistedreactor_test.py:
  - do conditional includes based on twisted
  - remove superfluous logging
  - don't use assertGreater
  - use threading instead of the thread module. Join threads at the end of test.
  - properly close the file descriptor in Reader/Writer's connectionLost().
  - make use of tornado.platform.auto's set_close_exec.
  - write only one in the pipe, and check for that.