Hearthpwn

Mulligan
Always keep:
Flame Imp, Kobold Librarian, Voidwalker, Void Analyst.
Consider keeping:
Soul Infusion - if you have Doubling Imp in you hand,
Demonfire - if you have Flame Imp or Voidwalker,
Knife Juggler - useful against aggro,
Vulgar Homunculus - as above.

Strategy
Deck is based on buffing your minions to take control of board early to quickly destroy your opponent. Thanks to some drawing cards and your hero power you'll almost never have empty hand. Buffing your demons is a key to win as they are hard to deal with.

Try to take board control early with 1- and 2-cost minions. Void Analyst is the most important here. After turn 3 or 4 you should have some minions on board to start rushing face. Keep in mind that you should also do key trades (eg. Silver Hand Recruits at turn 4). Further strategy is just playing minions in tempo and remember about opponents AoE. Also remember that Soul Infusion is useful not only with Doubling Imp  but also Doomguard which can be nice finisher.

Alternatives
The Soularium - good card in place of Mortal Coil or Kobold Librarian,
Crystallizer - synergy with Nethersoul Buster but imho 1/3 body is not worth synergy for one card.
Saronite Chain Gang - synergy with Soul Infusion, useful against aggro,
Dark Possession - instead of Mortal Coil.

Share your ideas and feedback in comments!