use cow instead of strings for html templating.

cow works best when you provide generator types. i.e.
types that are encapsulated by "t -> Html.t"
cow is able to generate such functions, however the html I
want to generate differs from the defaults so I wrote most
of them by hand.

As an added bonus it is really simple now to switch between
hypermedia inlined vs linked representations by optionally
providing arguments to functions that generate the html
generator data.
abstract out the tweep list html rendering.

Also added comments relating to the different ways
to change the html that gets generated for the hypermedia api.