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.
Added SpecialPolynomials

Legendre methods and Horner Scheme abstraction building.
Private marked some functions

irrelevant update, just to keep up to date across multiple machines
Special polynomials work

Added Special Polynomials with generic stream generators for legendre,
chebyshev and hermite polynomials
Also added two new oft used polynomials for x and 2x to Polynomial
object.
All special polynomials

Added laguerre polynomials. Added assertion === tests for special
polynomials too.
Tidy up special polynomials and tested

Tests verified for special polynomials
Merge pull request #139 from jamesthompson/topic/polynomials

Split polynomials into separate files and added Special Polynomials instances