Add parameter summary to hostclass html docs

Format cribbed from YARD method and attribute summary blocks. Still some work
to do, but initial results don't look that bad.
Extend doc generation to defined types

Puppet Yardoc now generates documentation for defined types in addition to host
classes. The actual change was fairly small, however this patch is quite large
as most of the host class bits were re-architected to inherit from defined
types. Host classes are basically the same as defined types with the addition
of inheritance. Pops also models Host Class using a similar inheritance
relationship.
Add feature check for backports under Ruby 1.8.7