Doc fixes

Couple of commits.  First one removes an unnecessary library dependency on utf8-strings.  It's always nice to get rid of unnecessary library deps.

The second commit makes changes to Text.Hastache docs so that the two code examples build out of the box.  The latter examples was missing a language pragma that's needed with Generics.  I also added OverloadedStrings, I think most folks working with Text strings use it.

This also removes an unnecessary Data.Generics import.. the example compiles without it.

It was surprisingly hard to get the syntax right for haddock, that's the reason for the messy escapes.

BTW - I wonder if it'd make sense to use GHC's Generics instead of 'syb'?  I'm not a heavy user of either, so don't know about the pros and cons of each approach.