Move Soundex test data to file
Use yml ext for YAML test data
Ignore non-A-Z in input to Soundex

Soundex is only defined for A-Z. Previously, coding an input containing
a space would give a result of nil.

Fixes issue #19.
Use Ruby's built-in string padding
Return nil for uncodable strings
Remove unecessary branch
Add licence to gem

Closes #20.
Constrain Rake version so gem is happy

Eliminates a warning (and probably makes things more reliable in
future).
Version 1.3.0

Adds maximum distance optimisation for Levenshtein.
Removes support for Ruby versions before 1.9.