Well. The easiest way to do it is having ursa in your team. Fury swipes attacks stack and stack and stack which would get you the roshan kill as fast as possible. The only problem remains to hang on enough in there with ursa until you kill Roshan. This means that if you go with your mates and you have someone to do some tanking for you, you should have no problem in killing him.
In dota 1 the easiest way was to go ursa/syllabear and summon the bear to do the tanking.
I don't know if Syllabear is yet implemented in dota 2 but if not it will probably be soon implemented. Even so, tanking can be improved with some armor bonus skills such as lich's frost armor, maybe a bassilius etc. Also you should consider leaving the last hit to only one of your team's players as in early game some heroes starting with lvl 5 and a half can just turn the game by themselves. Vampiric aura from LeoriC(Skeleton king) is also a viable option but it practically doesn't help one bit in killing it faster. So maybe go for some attack speed aura as 90% of the damage will be done by ursa's fury swipes so just try to land them as fast as you can.
Ok. Now that it's settled that it's very easy to do this with ursa and kinda any random tank(as item maybe go gloves of speed on ursa),let's think if it's possible to do so with any other heroes.
No. There you have it.
Without ursa I don't think it's possible to kill it before the creeps spawn, but this is an arguably +- 10-20 seconds marge of error.
In the "there is a chance to do it" category, pretty much any heroes that you can solo roshan at lvl 7 would increase your chances. These would be:Venomancer(Plague Wards)/Enigma(with rexx only be4 creeps spawn as you need to kill his pet to create minions as there aren't any minions be4 the game starts ofc)/Furion(Prophet-just spawn those trees) etc.
Also, if you play a semi-pro cough like game, probably you will need some smoke of deceit to get to Roshan undetected. Otherwise it's just gonna be sad.
Hope this was kinda helpful in helping you make an idea, if you need any more suggestions just ask(anything dota-related).
This being said, a more patiently written answer you will find here but it's from dota 1 so basically 99% of the stuff in there applies to dota 2 as well.