Rearranged order of documentation.

[marick]

Also added quick reference.

Addresses #142

Question: There are other packages whose documentation I want to tweak. Should I open issues, or just launch into pull requests?

For example, as my next project I'd like to move the book explanation I gave for Data.Tuple.Nested's Tuplen and Tn types into the module documentation.

[kritzcreek]

Just to let you know, the reason I haven't merged this yet is that we're moving this library together with sets into ordered-collections and because we restructured the modules somewhat I'll need to make sure that the ordering of exports persists through reexports.