User provided messages, or any valid expression given as second
argument to the assert statement, are now shown in addition to the
py.test introspection details. Formerly any user provided message
would entirely replace the introspection details.
Fixes issue549.
The only remaining 'py.test' references are:
* those referring to the 'py.test' executable
* those in code explicitly testing py.test/pytest module compatibility
* those in old CHANGES documentation
* those in documentation generated based on external data
* those in seemingly unfinished & unmaintained Japanese documentation
Minor stylistic changes and typo corrections made to documentation next to
several applied py.test --> pytest content changes.