a little less optimized, but no dirty tricks anymore

Make calc_levdist static and fix pointer swapping.

- English fix.

name changed from levdist() to levenshtein()
(although i bet we'll get a levenstein() alias some day)

added function "int levdist(string str1, string str2)"
that will calculate the Levenshtein distance between two
strings (faster and possibly more accurate than similar_text())