Added view focus to overlay text.

In both pixels and inches!
Fixed centering of view/perspective.

• Now centered on 0×0 rather than 0×0 being in the corner.
• Circle is now created at the world center, 0×0, rather than 0×-5.5.

Minor:
• Using intermediate change-only CGAffineTransforms for better understandability.
• outsideCirclePath and insideCirclePath accessors: `boundingRect` separated off for easier debugging.
Set starting view focus to (0, 7)

View focus is stored in inches (to correlate with ring size constants), and converted to pixels during construction.  It probably doesn't need to be reiterated, but this is messy and should be elegantly whisked away sooner or later.
Minor: Coordinates written with ‘,’ instead of ‘×’

I don't know why I was thinking “by” for coords.  It really only makes sense for sizes.

Changed in comments and drawViewFocusCoordsOnLine:within:.  drawViewFocusCoordsOnLine:within: is also using thin space between numbers and units now (http://en.wikipedia.org/wiki/Space_%28punctuation%29#Spaces_and_unit_symbols).