Add section on 'why create a new tool'
Add initial section on Mocha helpers.
Per nevir, remove line on selenium testing.
Add justification for HTML tests.
Clarify creation of implicit suites for test files
Add missing step to creating tests guide.
Clarifications for assertion styles setup
Update Platform.flush to flush()
Update plaform.js > webcomponents.js
Add note about testing with polymer-ready
Add section on unit testing helpers
Add missing `done` for assertion to work
Adjust to use nevir's wording suggestion for tutorial