Enhance wording: replace 'instances' with 'parts' in description of unit tests

This is in line with wording using in the API description in the README.
Simplify explanations in within.js, taking advantage of details in README

It is now safer to describe functions in within.js with a broader brush
since nuance has been introduced in README.md.
Compare context objects with module data objects module1 and module2 in tests
Return shortcut object when calling within() without callback

The shortcut object contains the four method get, set, publish, subscribe.

This feature has already been described in the API section of README.md.