Add dynamic fee support for protocol and swap fees

[AugustoL]

This PR implements the smart contract changes discussed in issue #1.

Changes in TokenPair factory:

• Adds protocolFee public variable, used as divider for the swap fee. protocolFeePercentage = swapFeePercentage * 1/protocolFee+1 changes

    uint8 public protocolFee = 5; // uses 0.05% (1/6 of 0.30%) per trade as default

• Adds setProtocolFee and setSwapFee functions to token pair factory, they can be executed only by the feeToSetter address.changes

    function setProtocolFee(uint8 _protocolFee) external {
        require(msg.sender == feeToSetter, 'UniswapV2: FORBIDDEN');
        require(_protocolFee > 0, 'UniswapV2: FORBIDDEN_FEE');
        protocolFee = _protocolFee;
    }

    function setSwapFee(address pair, uint8 swapFee) external {
        require(msg.sender == feeToSetter, 'UniswapV2: FORBIDDEN');
        IUniswapV2Pair(pair).setSwapFee(swapFee);
    }

• Adds swapFee public variable in token pair contract using 0.3 % as default.(changes)[]

    uint8 public swapFee = 30; // uses 0.3% fee as default

• Adds setSwapFee function in the token pair, which can be called only by the pair factory contract. changes

    function setSwapFee(uint8 _swapFee) external {
        require(msg.sender == factory, 'UniswapV2: FORBIDDEN'); // sufficient check
        require(_swapFee <= 30, 'UniswapV2: FORBIDDEN_FEE'); // fee percentage check
        swapFee = _swapFee;
    }

• Swap fee denominator is 10000 instead of 1000, allowing use two decimals swap fee. changes
• Get protocolFee to be used in token pair form pair factory. changes

Added two test cases, one that tests the new protocolFee after changed and another that tests the swapFee after changed in #6

[AugustoL]

Tests added for different being used, on each test case multiple trades are executed and we check the right protocolFee is transferred in the next liquidity change.

[AugustoL]

Next question is the protocolFee variable, should we change the name to something that implies that is the value for whats the swapFee is divided? Like swapFeeDivider and explain with a comment in the contract how to calculate the protocolFee based on that.
@jpkcambridge @pimato

[pimato]

@AugustoL what about protocolFeeDivisor ?

[AugustoL]

@AugustoL what about protocolFeeDivisor ?

protocolFeeDivider ? ^^

[AugustoL]

@pimato We will leave the change of the protocolFee for the next PR.

[AugustoL]

@pimato We will leave the change of the protocolFee for the next PR.

I just changed it here for protocolFeeDenominator