rename Writer to Client

[suyash]

The fundamental reason for doing this is that the name writer in the golang community implies to most that the type implements io.Writer, and initially it did, but later on all the writing capability was abstracted away into bytebuffer, and the Buffer type does implement io.Writer, but speed.Writer doesn't, and I don't think the name is apt anymore. Looking at the current definition of the interface, I think Client is a more appropriate name, and will probably create less confusion.

[natoscott]

@suyash the suggested change certainly makes sense to me, go for it.