Add parent node to the emitter
Add parenthesis around binary connectives conditionally
Add SQL::Generator::Emitter::Root null object

* Change parent to refer to the parent emitter
* Replace nil checks with null object
Refactor parenthesize? test to be simpler
Fix flay, flog and reek thresholds
Merge pull request #11 from dkubb/add-parent-node

Add parent node