Generic popover in place.

I'm using a cocoapod called `WYPopoverController` for the popover that has a very simple API luckily. I've subclassed it so we can customize it globally, that subclass is called `ZSPopoverController`.

The two things of note here are `ZSExpressionOptionsTableViewController` and then the new code in `ZSCodeEditorSetStatementTableViewCell.m` where I'm setting up the popovers. A similar design pattern can be used to make a `ZSStatementOptionsTableViewController` for the plus-sign stuff (`if`, `set`, `move`, etc) and probably as `ZSEventOptionsTableViewController` for choosing events.

/cc @vladimirzhidkov