Added support for serialized "collect_params".

This is option 5 to collect_params, and when enabled will also add serialized
return values into the machine readable traces as an extra line with type "R".
Fixed existing tests to the new format version number.
Solve the issue with closures by hijacking the serialize handler.
Make sure we don't break < PHP 5.3
Fixed ZTS mode.
Make sure we don't serialize NULL.
Added serialized support in stack traces too.
Added test cases for the serialized format and stack traces.
Fixed test cases. It's "xdebug.collect_return" and not "xdebug.collect_returns".
Merged pull request #79