* 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