Add :parent_example_group to RESERVED_KEYS.

It's managed by us so users shouldn't pass that metadata.
Merge pull request #1468 from rspec/update-reserved-keys

Add :parent_example_group to RESERVED_KEYS.
[fix rspec/rspec-core#1433] Added option to disable all monkey patching

- Added option to disable all monkey patching
- Fixed syntax to support ruby-1.8.7
- Refactored disable_monkey_patching!. Added docs, feature.
- Moved monkey patching specs to proper place
- Fixed zero monkey patching feature to support ruby-1.8.7
- Re-written specs to be more BDD-ish.
- Re-written docs for disable_monkey_patching!
- Made non-configured frameworks specs easier
- Fixed cuke to be lighter
- Fixed specs to support 1.8.7
- Improved rspec_<lib>_loaded? a bit;
- Improved not configured frameworks specs.
- Fixed typo in comments-docs.