Golang x text encoding

This commits remove the mahonia module and replaces it with the standard functions of the Golang Sub-repositories http://godoc.org/golang.org/x/text/transform and http://godoc.org/golang.org/x/net/html/charset (which are using http://godoc.org/golang.org/x/text/encoding)

This is just one suggestion, I am happy to implement things differently if we want to keep the mahonia module in some form.
This is one way. We can also just decide to return the original content if there was a decoding error