Test display_exception_details

Part of #272
Test display_exception_details with causes

Part of #272
Prevent infinite recursion via Exception#cause

It is possible the causes of two exceptions to refer to each other.
When displaying the causes to the user rake would raise a
SystemStackError trying to show the causes repeatedly.

Now rake looks for seen causes and stops when it sees the same cause a
second time.

Fixes #272
Fix some typos
Merge pull request #275 from jwworth/task/improve_documentation

Fix some typos
Add #275 to History
Set release version and date
Add Avdi Grimm's rake episodes 5–7 to README
Fix some typos