Document how to stub route helper functions in isolated tests. Addresses #506.
Improve documentation about stubbing route helpers

* Get rid of an unnecessary newline
* Change 'include Rails' to 'require Rails'
* Clarify when you would need to explicitly include ViewHelpers
* Switch from mocha `stubs` to RSpec `stub`
Merge pull request #513 from carols10cents/route_stubbing_documentation

Document how to stub route helper functions in isolated tests. Addresses...