Calculates the Levenshtein distance between two strings.
The Levenshtein distance is defined as the minimum amount of
single-character edits (either removing a character, adding a
character, or changing a character) necessary to transform a source
string into a target string.

Input:
    source: The string you begin with.
    target: The string to transform into.

Output:
    The Levenshtein distance between the source and target.

Example:
    "electron" can be transformed into "neutron" by removing the
    "e", turning the "l" into "n", and turning the "c" into "u".
    >>> levenshtein("electron", "neutron")
    3