Add the Bounties pallet from FRAME

[lsaether]

We would like to start using on-chain processes for rewarding members of the community. Parity provides a couple standard pallets related to treasury management and spending, the main Treasury pallet is already added to our chain. We can extend the treasury functionalities by adding the Bounties pallet. This would allow us to use the on-chain treasury for funding community work instead of handling it in the current opaque, off-chain way.