document how to preload apps in startup.pl

Add instructions for using a startup.pl file, checking the restart_count
to make sure the app doesn't load twice (thanks to avar for that info.)

Also document how to catch errors from app loading so that the server
still starts, how to detect dynamically loaded modules so that they
can be preloaded and add paths to @INC.
Merge pull request #357 from rkitover/master

startup.pl instructions for mod_perl