Add string encoding awareness.

Raised by Matthias Kadenbach in issue #18, RE2 should return strings
with either UTF-8 or Latin 1 (ISO-8859-1) encoding according to the
:utf8 option flag.

This sets the encoding appropriately according to that flag on the
pattern thanks to Aaron Patterson's "String Encoding in Ruby 1.9 C
Extensions",
http://tenderlovemaking.com/2009/06/26/string-encoding-in-ruby-1-9-c-extensions.html
Remove commented Rake task options.