Update readme
Add sender testbench
Refactor checksum, receiver

- Split checksum into two modules, one for ones complement sum and one
  for actual checksum (just negation of ones comp. sum).
- Refactor receiver to actually correctly check if data is uncorrupted
  by adding a recalculated ones comp. sum to the appended checksum to
  see if it's all 1's (rather than just comparing equality of newly
  calculated checksum with appended checksum)
- Refactor receiver testbench
Add Makefile
Rename readme