* 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 371fe3e567e0ec894caa8d5f14ad590b18c5ae86.

* 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