Fix rounding error issue in tapEth calculation

[Mars12-dev]

One can get a rounding error when executing function transfer or function burnSharesFrom from the contract TapETH, when the input _tokenAmount is small.

Exemple
let's consider the following exemple:

• totalSupply: 189804710086258501256 wei

• totalShares: 189804710086258501256 wei

• The Balance of Bob in tapETH is 0 wei.

• Alice sends25 *10^18 weitapEth to Bob. Bob receives 24999999999999999999 wei tapETH. So, there is a rounding error of 1 wei tapETH.

• Now, Bob burns his total balance of tapETH (24999999999999999999 wei). Bob keeps 1 wei tapETH and 1 wei shares. So, there is a rounding error of 1 wei.