Test multiple Swaps for a single buyer

[lukem512]

This has been prevented with #8a7fc554056928b7ce6b97cca529278763b110fc but there is not yet a test in place. The following code requires modification to check for the a failed require, which will eventually map to a REVERT opcode.

it("user cannot create a second Swap whilst one exists", function() {
    return ts.create(token.address, amount, price, seller, buyer, {from: buyer})
    .then(function() {
      return ts.create(token.address, amount, price, seller, buyer, {from: buyer})
    }).then(function() {
      // TODO: check for REVERT
    })
  });