Add in the ability to enable / disable performance_schema

From MySQL 5.6.6 it was made enabled by default, while previously it was disabled. It has overhead to it that not everyone will desire, so turning it off for everyone by default in the cookbooks.
fix `performance_schema` feature:

* fix typo in `my.cnf` templates
* add basic chefspec expectations
* update the README with new attribute
add contribution from @helgi to the README
Merge pull request #231 from phlipper/performance_schema

Add support for managing the `performance_schema`