add Hashable instance for Options SVG R2 (and remove Show)

unfortunately we also have to add some orphan Hashable instances for
blaze-markup types.  We could contribute them back upstream to
blaze-markup, but that would require adding a hashable constraint.
actually remove Show (Options SVG R2) instance
clean up warnings
Don't emit last segment of a loop if it is linear

See diagrams/diagrams-cairo#38.  This wasn't actually causing any
observable problems in the SVG backend output, but this seems a
better/more robust way to do things in any case.
Merge branch 'master' into builder-cleanup
Merge pull request #45 from diagrams/builder-cleanup

Add Hashable instance for Options SVG R2
CHANGES for 1.0.1
write Hashable (Options SVG R2) instance manually

Work around a bug in GHC 7.4.2, which chokes when deriving Generic
instances for associated data types.
typo fix
Release notes for 1.0.1.1
Allow lens-4.0
allow blaze-markup-0.6
Release notes for 1.0.1.2
Merge branch 'master' into units
master -> units