Typo noticed while attempting to trace fastcgi mouseover issue.
Modifications to allow pre-loading of datasources to be configurable
via GBrowse.conf.

Issue is that pre-loading of the large numbers of datasources can result in endless
start up of fastcgi processes until no memory remains on server.
Replicated changes from gbrowse needed to prevent pre-load of datasources to gbrowse_img
Merge pull request #32 from keiranmraine/master

Makes pre-loading of datasources under fastcgi a configurable option