Fixed a bug where permalinks from one blog referred to another.

I had misunderstood the behavior of named Subrouters in the Gorilla mux
library. Blogs now must have a unique name so their named routes are
unique from one another if they are created on Subrouters of the same
parent Router.

A number of previously exported members of Blog are also now private;
some will probably be exposed through methods at a later time.
Added a TODO list.