vstart documentation + bugfixes

In Order to make contribution easier for newly developers, i documented the vstart.sh script,
with a manpages, and added a missing option to the usage message.
As i encountered problems using it at start, i thought some documentation would be welcome so that developers can know there is a simple way to test their code locally.

I also added a small fix to prevent a segmentation fault if the rest connection was not established (call to member method on null pointer). Not sure if it should happens, but i think it is because when you have a freshly installed cluster, it doesn't have a region map, so the rest_conn object contained in the store object is not properly initialized.