Improve customizability of PTKView

This pull request cleans up `PTKView` and adds the following features:

1) Size-independent layout. You should be able to specify any reasonably-sized frame and PTKView will attempt to lay out in that area, rather than having hardcoded sizes everywhere with no layout code.
2) Configurable font and font colors.
3) Ability to override `backgroundImageView`.
4) Ability to supply a custom `separatorView` (e.g. for a vertical line instead of a gradient).
5) Ability to supply custom padding via `contentInsets`.

The defaults should be the same, but please let me know if you notice any differences.

This work was done because we had a design for which `PTKView` was a great fit, however I was forced with the choices of: forking off our own and customizing it heavily; writing our own replacement; or updating `PTKView` as in this PR to allow customization hooks in the hopes that others will find it useful.