Nodoc internals

As a first step in defining the 1.0 API, I decided to hide from the API documentation all the stuff that I'd consider "Prawn internals", i.e. the things that we won't promise any stability for and don't expect end users or extension authors to use.

It may be easier to review this pull request by looking at the classes that I *didn't* mark private. You can find a snapshot of them here:

http://prawn.majesticseacreature.com/api_preview/index.html

For everything that remains, we'll go through on a method-by-method level and mark things in one of four ways:

* Public stable API (these will use semver)
* Public experimental API (these will be subject to change on each minor release)
* Extension API (these can change on each minor release, but we'll display warnings for a release or two first)
* Internals (These are hidden from the documentation entirely, as in this pull request)

The changes in this pull request are all related to that last point!

Feedback is welcome, if you want to argue something that isn't currently marked internal should be, or vice-versa.
(Note, we're only at the class-level granularity right now)