spksrc: Add mercurial/hg download method

Adds mercurial/hg download method to spksrc.

Example syntax:
PKG_NAME = syncserver
PKG_VERS = 1.0
PKG_EXT = tar.gz
PKG_DOWNLOAD_METHOD = hg
PKG_HG_REV = tip || ba4bf783018f || rpm-1.0-9	#supported: tip, revision id, tags
PKG_DIST_NAME = server-full
PKG_DIST_SITE = http://hg.mozilla.org/services
PKG_DIST_FILE = $(PKG_NAME)-r$(PKG_HG_REV).$(PKG_EXT)
PKG_DIR = $(PKG_NAME)-r*