Import "code.google.com/p/go.text/encoding" instead of "code.google.com/p/mahonia".

mahonia is deprecated on 2013-12-26.
https://code.google.com/p/mahonia/source/detail?r=937b608805fa1ef555f3247a411a967ca3fa3466
Merge remote-tracking branch 'origin' into use-go.text-encoding
Merge pull request #8 from oov/use-go.text-encoding

Import semi-official package instead of mahonia.