Bug 883007 - Build error while trying to backup webapps directory after building gecko

Directories to be prereserved are managed via variable "PRESERVE_DIRS"

Change-Id: I1367f475fd4fcc8356823012872b6a17305dacbf
Merge pull request #98 from pchangbin/master

Bug 883007 - Build error while trying to backup webapps directory after building gecko