follow $MANPATH directory structure

[sunaku]

gem-man doesn't follow the standard $MANPATH directory structure (man/manX/Y.X where X is the section number and Y is the manual page) but instead uses a single flattened directory (man/Y.X) to keep manual pages from all sections.

For example, observe how man(1) cannot find bar.3, whereas it can find foo.1:

$ mkdir -p man/man1
$ touch man/man1/foo.1
$ touch man/bar.3
$ man -M ./man -a bar
No manual entry for bar
exit 16
$ man -M ./man -a foo     # success!

This prevents us from adding man/ subdirectories in installed gems to $MANPATH and using plain old man(1) thereafter to look up both system and gem manuals.

Thanks for your consideration.

[sunaku]

I've added a commit to fix this issue. Please review, merge, and release.