Reimbursment Pool Setup and Scaffolding

[apbendi]

• [x] Create Reimbursement Pool contract and test harness
• [x] Deployed with following params, stored as public immutable properties
  • [x] Reimbursement token (should sanity check interface on deploy)
  • [x] Treasury Token (required)
  • [x] Collateral Token (optional, i.e. can be 0x0)
  • [x] Maturity Date (unix timestamp, must be in the future)
  • [x] Maturity exchange rate