Fix variable scope
Remove unused variable declarations
Add missing semicolons
Use jsxhint and fix all warnings/errors
Fix all JSHint warnings in examples
[added] Router.History

[changed] goBack returns a boolean value

Users can use Router.History.length to determine if the router has
any history. router.goBack() is a no-op when the router does not
have any history and emits a warning.

Fixes #408
Use History