let user change store root at runtime

I wanted to put up a web application on [OpenShift](http://openshift.com/) with a 4store database behind it. But on OpenShift every part of the application (including the database) has to store its data under a certain directory, which has a random name assigned when the app is created, like /home/2793923abec37b37bc3e. So I changed the storage path from being nailed down at build time to having a default set at build time, which can be overridden at run time using an environment variable FS_STORE_ROOT.