Added spire.math.poly

Split up Polynomials.scala :
Taken PolyDense and PolySparse and added to separate files in
spire.math.poly
Added Term to PolyTerm.scala in spire.math.poly
Created SpecialPolynomials object in spire.math.poly -> I would like to
add generators for certain special polynomials in here in the near
future.