add missing option instances
add metric space syntax
add CMonoid, a commutative monoid
Add CRing[A].
Merge pull request #155 from non/topic/commutativity

Topic/commutativity
first pass at a spire guide
fix some typos in GUIDE.md
Test deps for scala{test,check} for spire-macros.
small guide update
Added Polynomial.interpolate and cleaned up return types.
Merge pull request #7 from non/master

Updating with new Polynomial stuff prior to hacking!