Remove unnecessary import for twisted.python.log.addObserver from log.py.

 * It gets set at the end at of the module by copying BridgeDBLogPublisher's
   methods to the module level.
Remove unnecessary code from log.configureLogging() function.

 * Eventually, this whole function should be removed.
Add function Main.changeToRunDir() for changing runtime directories.

This function will:
 * Use the current working directory if RUN_IN_DIR isn't set.
 * Reset the RUN_IN_DIR config setting to the expanded, normalized path of
   whatever RUN_IN_DIR was set to.
 * Change to that directory.
If unset, or set to None, we should catch the resulting TypeError.
fixup! Add function Main.changeToRunDir() for changing runtime directories.

 * We can't actually do this. Duh. The $PWD resets to the parent's when we
   exit the local scope.
Add function Main.beginLogging for configuring the logger.