Move to new query builder

[allejo]

Our query builder is great. However for more complicated queries that I've been writing lately, it becomes really difficult to manage things. Our current query builder is at the core of the project, so changing it will not be easy.

Since our models are extremely complicated, this isn't a job for Doctrine; yet, at least.
I've been looking at different options and have found Pixie to be plausible solution and it has support for nested queries.

My approach to this would be to convert our current QueryBuilder class into a wrapper for Pixie. This would allow us to keep all of the current QueryBuilder instances across the project and it'd let us build shortcuts for common tasks.