StandardToken encapsulation (#1197)

* make StandardToken state variables private

* simplify mocks

* document new internal functions

* fix link to ERC20 document

* revert order of Transfer and Mint events

* Revert "simplify mocks"

This reverts commit 371fe3e567.

* add tests for new internal functions

* add check for null account

* add checks for balances and allowance

* add inline docs to BurnableToken._burn

* remove redundant checks and clarify why

contracts/examples/SimpleToken.sol

@@ -22,9 +22,7 @@ contract SimpleToken is StandardToken {
    * @dev Constructor that gives msg.sender all of existing tokens.
    */
   constructor() public {
-    totalSupply_ = INITIAL_SUPPLY;
-    balances[msg.sender] = INITIAL_SUPPLY;
-    emit Transfer(address(0), msg.sender, INITIAL_SUPPLY);
+    _mint(msg.sender, INITIAL_SUPPLY);
   }
 
 }