auto versioning

If recipe points to not fixed version from git:

SRC_URI = "git://example.com/project.git;protocol=ssh;branch=${SRCBRANCH}"
SRCBRANCH = "devel"
SRCREV = "devel"

then version needs to be update for every change of source code. To detect change automatically and prevent sstate errors:

SRCREV = "${AUTOREV}"
PV = "1.0+git"

To automatically increase package version number.