module: escrow module

Simple escrow module similar to the one in pooltoy [here](https://github.com/interchainio/pooltoy/tree/escrow/x/escrow). More extensive ADR can be seen [here](https://github.com/interchainio/pooltoy/blob/readme/docs/adr-escrow.md). This would allow very basic OTC trades that could be P2P, P2-community-pool or community-pool-to-community-pool using interchain accounts.