GetSegment wrapper with Parametric and EndValues instances

For extracting individual segments from trails by parameter.  This
commit only includes instances for lines, need to add loops and
trails.
D.Trail: Parametric and EndValues instances for GetSegments on loops and tails
D.Trail: document semantics of Parametric GetSegment at segment junction
D.Trail: add reparameterization iso to result of Parametric GetSegment
new module D.Tangent: computing tangent + normal vectors
travis: turn on Haddock builds
Merge pull request #113 from diagrams/tangent

API for computing tangent and normal vectors to segments and trails
Fixed haddocks in the only way I could.
Tied the offsetSegment epsilon to radius.

The default epsilon was much to small for typical uses of offset.  But
it isn't clear what concrete epsilon to use.  Now epsilon is relative to
the given radius.  You can read it as a parameter that lets you get an
offset "within epsilon percent" of the true offset.

This does not quite fix #118.
D.Prelude: export Diagrams.Tangent
Merge branch 'master' into arrows
D.TwoD.Arrowheads: get rid of spurious Haddock comments