Lots of doc reorganization

The changes in this branch are:

* Merge CMOP docs into Moose where there are corresponding meta classes (so now all Attribute docs are in Moose::Meta::Attribute, for example)
* Converted POD lists into =head2 or =head3 (as appropriate) so that individual methods show up in the TOC on MetaCPAN.
* Rewrote the exception doc generator more or less from scratch. This removed a lot of cruft.

There's also some other small doc fixes in here too.