Replace CopyOnWriteToken (old code) with PreserveBalancesOnTransferToken (new code)

[AnthonyAkentiev]

Current code (old code) is here

https://github.com/Thetta/Thetta-DAO-Framework/blob/master/contracts/tokens/CopyOnWriteToken.sol

https://github.com/Thetta/Thetta-DAO-Framework/blob/master/test/tokens/copy_on_write_token.tests.js

New code is here (OpenZeppelin pull request)

https://github.com/Thetta/openzeppelin-solidity/blob/master/contracts/token/ERC20/PreserveBalancesOnTransferToken.sol

https://github.com/Thetta/openzeppelin-solidity/blob/master/contracts/mocks/PreserveBalancesOnTransferTokenMock.sol

https://github.com/Thetta/openzeppelin-solidity/blob/master/test/token/ERC20/PreserveBalancesOnTransferToken.test.js

New solution is better and more generic.
Use it instead of CopyOnWriteToken in StdDaoToken.
Also, you can use new SnapshotToken wrapper to get balances.

Description

So now we have 2 usage scenarious:

One

PreserveBalancesOnTransferToken token;

token.mint(ADDRESS_A, 100);
assert.equal(token.balanceOf(ADDRESS_A), 100);
assert.equal(token.balanceOf(ADDRESS_B), 0);

SnapshotToken snapshot = token.createNewSnapshot();
token.transfer(ADDRESS_A, ADDRESS_B, 30);

assert.equal(token.balanceOf(ADDRESS_A), 70);
assert.equal(token.balanceOf(ADDRESS_B), 30);

// see here. snapshot is ERC20 compatible, but transfers are locked
assert.equal(snapshot.balanceOf(ADDRESS_A), 100);
assert.equal(snapshot.balanceOf(ADDRESS_B), 0);

token.stopSnapshot(snapshot);

Two

PreserveBalancesOnTransferToken token;

token.mint(ADDRESS_A, 100);
assert.equal(token.balanceOf(ADDRESS_A), 100);
assert.equal(token.balanceOf(ADDRESS_B), 0);

uint someEventID_1 = token.startNewEvent();
token.transfer(ADDRESS_A, ADDRESS_B, 30);

assert.equal(token.balanceOf(ADDRESS_A), 70);
assert.equal(token.balanceOf(ADDRESS_B), 30);

assert.equal(token.getBalanceAtEvent(someEventID_1, ADDRESS_A), 100);
assert.equal(token.getBalanceAtEvent(someEventID_1, ADDRESS_B), 0);

token.finishEvent(someEventID_1);