Dialog

Work in progress to support Dialog by adding a "center" orientation argument to delite/place and delite/popup (and thus to HasDropDown).

Builds on HasDropDown changes from #307.

Still needs:

1. code for repositioning and resizing the dialog when the focused node changes (i.e. when the keyboard is shown/hidden), and when the viewport changes size (phone rotated).   Maybe expand on _repositionAll().
2. add test for auto-sizing (large centered node)
3. create DialogBase class: tab handling (looping from last focusable element to first)
4. modal support (using DialogUnderlay, plus some code from DialogLevelManager

> This method (popup.open()) used to always return an object. This is no longer the case, 

OK, that's a typo, I'll fix it.

> Additionally, popup.center() must also return an object, just as popup.around()/at().

Why do you think it should return an object, and what object would you like it to return?  You mentioned the code at https://github.com/wkeese/delite/blob/dialog/HasDropDown.js#L484 but that's for when autoWidth == true or forceWidth == true, and those options make no sense with an orient==center dialog.