Add 'goedelise' program

The goedelise program goedelises arbitrary ASCII strings into numbers (and
can then decode these numbers into the corresponding strings).

It works like this: Let c be the n-th character in the string (beginning
with 1). The value of c is the ASCII value of the character. Then set
x_n = p_n^(c+1) where p_n is the n-th prime. Set g = x_1*x_2*... the product
of all such x. g is then the desired, unique Goedel number. That this
function (and its inverse) is a bijection is easily proved by the
fundamental theorem of arithmetic.

Add an assertion to get_nth_prime() which fails when n equals zero where the
function is not defined.
Merge branch 'master' of https://github.com/tboege/sieve

Conflicts:
	naive/.gitignore
	naive/Makefile