Add prayer to Dagr.

This prayer from Sigrdrífumál[0] is sometimes considered to be the only Norse
prayer we still know of that hasn't had Christian alterations made to it.

[0]: http://en.wikipedia.org/wiki/Sigrdr%C3%ADfum%C3%A1l#The_valkyrie.27s_drinking-speech
[1]: http://en.wikipedia.org/wiki/Prayer#Germanic_paganism