Print help when no args given and no fabfile.
Print help when no args given and no fabfile.

Fixes #75.

akaariai/fabric,xLegoz/fabric,rbramwell/fabric,getsentry/fabric,ericholscher/fabric,rodrigc/fabric,kmonsoor/fabric,opavader/fabric,askulkarni2/fabric,pashinin/fabric,ploxiln/fabric,StackStorm/fabric,fernandezcuesta/fabric,bitmonk/fabric,rane-hs/fabric-py3,likesxuqiang/fabric,amaniak/fabric,jaraco/fabric,itoed/fabric,raimon49/fabric,SamuelMarks/fabric,sdelements/fabric,cmattoon/fabric,MjAbuz/fabric,cgvarela/fabric,tolbkni/fabric,haridsv/fabric,qinrong/fabric,tekapo/fabric,kxxoling/fabric,mathiasertl/fabric,hrubi/fabric,bspink/fabric,pgroudas/fabric,elijah513/fabric,felix-d/fabric,bitprophet/fabric,TarasRudnyk/fabric