depositERC20 should accept an _amount argument

[skmgoldin]

depositERC20 should accept an _amount argument that specifies some number of tokens less than or equal to the amount the DMS is approved to transferFrom. It should revert if the specified amount of tokens cannot be transferred. The new function signature should be:

depositERC20(address _tokenAddr, uint _amount) public

Please include test coverage in your PR!