My DD has just started doing exactly the same thing since yesterday!! I call it dragging rather than crawling but she's on the move and I have just spent all morning child proofing the lounge room!
As far as sitting goes -I read somewhere that sitting is not a developmental stage- it is something that all bubs do - so you don't need to worry! Rolling, crawling, pulling up then walking is the basic developmental frame - and sitting occurs in there whenever they are ready! Congratulations and enjoy your DS!