Add x^n example

[ryanlevy]

Example code will make a polynomial f(x) = ax^n for n>=0 (so naturally makes the const function as well).

(I always feel like I write ugly code so feel free to suggest stylistic changes)

Closes #3

[ryanlevy]

Just need to make the elementary functions version (and test) still

[JoeyT1994]

Closed and archived for a more general polynomial constructor.