Support arbitrary page sizes

[adamwulf]

When importing documents, the user should be able to import PDF pages of any bounds size and still be able to see the whole page. as of #1614 we just aspect fill the PDF into the iPad screen size. Instead, I should aspect fit the PDF and allow the user to zoom into the page w/o it snapping back to 1.0 (related to #1557)

For PDF size, I should also map the PDF to its physical printed size.

http://stackoverflow.com/questions/11809133/default-paper-size-and-unit-for-pdf-documents-on-ios

PDF and PostScript use "PostScript points" as a unit. A PostScript point is 1/72 inch. So the default page size is

612 x 792 points = 8.5 x 11 inch = 215.9 mm x 279.4 mm
This is the US Letter paper size.