strip leading slash from renter paths
strip leading slash from renter paths

as discussed in #894

MrStrong/Sia,Mingling94/Sia,ericflo/Sia,mnsl/Sia,ChrisSchinnerl/Sia,NebulousLabs/Sia,mharkus/Sia,NebulousLabs/Sia,rivine/rivine,ericflo/Sia,johnathanhowell/Sia,rivine/rivine,mtlynch/Sia,mharkus/Sia,ChrisSchinnerl/Sia,MrStrong/Sia,mtlynch/Sia,mnsl/Sia