Payroll: Add reentrancy guard to transfers loop

[facuspagnuolo]

From @sohkai (see here)

We should add a reentrancy guard to this function and Payroll.determineAllocation() (or perhaps all the functions with the employeeMatches modifier).
Right now, with ETH, you can re-enter into Payroll.determineAllocation() and modify your allocation so that a later token becomes 100% of your allocation. The worse case of this is if you set ETH to 100%, and then upon paying ETH, your contract re-enters into Payroll.determineAllocation() and sets a later token to be 100%.