consolidated release versioning

- PlotDevice-Info.plist has only placeholder version numbers
- the official version is pulled from plotdevice.__init__.py
- the build version is the commit number with “r” prepended
- for dist builds, version numbers are spliced into the Info.plist
after xcode finishes
- non-dist builds use the placeholder version string
better handling of importing the module in-place

- made the `you’re running from the source dist, build it first’ a
little clearer
- gives more information if the extensions fail in the general case