bitbake/lib/bb/fetch/*: Implement special 'now' handling for BitBake

bitbake/lib/bb/fetch/*:  Implement special 'now' handling for BitBake
    Patch courtsey Justin Patrin to teach BitBake SRCDATE="now" handling.
    If SRCDATE is set to now we will not look at the temporary DL_DIR
    for a saved tarball as we want to download a new version. OpenEmbedded
    should consider stamping the do fetch phase.

    Justin thanks for the patch and sorry for all the trouble

bitbake/doc/manual/usermanual.xml: Document the 'now' method for svn,svk and cvs