Monitor memory usage with each file processed by FileLoader::executeActions()

Abort early if usage might exceed the PHP memory limit.  Resolves pkp/pkp-lib#245.
I think I added some clarity for the memory pain points in the new commit (https://github.com/ulsdevteam/pkp-lib/commit/ef6153dd88e1b9a4b5a7f62f07537da1090fee91), though I've still missed one or more.  If the SMTP Mailer is used, ScheduledTask::_executionLog, ScheduledTask::execute() [with the return from $this->_getLogMessage()], Mail::_data['body'], Mail::send() [with the local $body variable], and SMTPMailer::mail() [with the exploded $lines] are all in the stack simultaneously.  Each of these will be a variation of the full execution log.

I like the idea of pushing the execution log to a file instead of a memory structure.  I suspect that this could be helpful even if the log file is just attached to the email, rather than inlined as the body.

I was also unfamiliar with the copyAccessLogFileTool; I'll look at that.