docs: mention .asCallback instead of .exec
docs: mention .asCallback instead of .exec

Reflects deprecation warning on 0.8.0: "bookshelf.exec has been deprecated, please use bookshelf.asCallback instead"
dougshamoo/bookshelf,shadedprofit/bookshelf,jadengore/bookshelf,tgriesser/bookshelf,stridentbean/bookshelf,apla/bookshelf,zdespolit/bookshelf,jadengore/bookshelf,cspotcode/bookshelf,shadedprofit/bookshelf,lanetix/bookshelf,stridentbean/bookshelf,tbutman/bookshelf,zubeio/bookshelf,stonegithubs/bookshelf,offlinehacker/bookshelf,zubeio/bookshelf,sxhao/bookshelf,joshturn/bookshelf,seegno-forks/bookshelf,TinOo512/bookshelf,rbong/bookshelf,tjwudi/bookshelf,rbong/bookshelf,TNuzzi/bookshelf,jordansexton/bookshelf,sanchesic89/bookshelf,vellotis/bookshelf,patbenatar/bookshelf,TNuzzi/bookshelf,princesust/bookshelf,shaunstanislaus/bookshelf,offlinehacker/bookshelf,sanchesic89/bookshelf,jadengore/bookshelf,vellotis/bookshelf,chrisbroome/bookshelf,patbenatar/bookshelf,TNuzzi/bookshelf,tjwudi/bookshelf,joshturn/bookshelf,chuym/bookshelf,shadedprofit/bookshelf,stonegithubs/bookshelf,chuym/bookshelf,shaunstanislaus/bookshelf,nodeonly/bookshelf,chuym/bookshelf,dougshamoo/bookshelf,zubeio/bookshelf,lanetix/bookshelf,sxhao/bookshelf,skevy/bookshelf,offlinehacker/bookshelf,patbenatar/bookshelf,skevy/bookshelf,vellotis/bookshelf,mdboop/bookshelf,apla/bookshelf,VideoAmp/bookshelf,sanchesic89/bookshelf,seegno-forks/bookshelf,VideoAmp/bookshelf,skevy/bookshelf,stonegithubs/bookshelf,tgriesser/bookshelf,dougshamoo/bookshelf,lanetix/bookshelf,tbutman/bookshelf,princesust/bookshelf,chrisbroome/bookshelf,bookshelf/bookshelf,princesust/bookshelf,nodeonly/bookshelf,MadcapJake/bookshelf,zdespolit/bookshelf,nodeonly/bookshelf,tbutman/bookshelf,zdespolit/bookshelf,TinOo512/bookshelf,sxhao/bookshelf,mdboop/bookshelf,jordansexton/bookshelf,bookshelf/bookshelf,VideoAmp/bookshelf,stridentbean/bookshelf,tjwudi/bookshelf,shaunstanislaus/bookshelf,MadcapJake/bookshelf,mdboop/bookshelf,joshturn/bookshelf,cspotcode/bookshelf,MadcapJake/bookshelf