discussion: reduce number of packages in the sdk

[LukeWinikates]

I think this sdk should have a single public package, and that package should be named wavefront.

I've noticed that telegraf imports only the senders package from this repo, and aliases it to wavefront

import (
    ...
	wavefront "github.com/wavefronthq/wavefront-sdk-go/senders"
)

that means they create a new sender by invoking wavefront.NewSender, which I think looks better than senders.NewSender

I think we could consider moving the exported members from senders, histogram, and event into a wavefront directory/package, and move most of their unexported members into internal.

This would be a big change, but I do think it would make the API of the SDK easier to learn and so I'd like to consider changing this before v1

[LukeWinikates]

Looking at #143 , I have a couple of new thoughts on this:

• wavefrontSender has a large implementation. I think we could move that type into an internal subpackage
• the histogram, event, etc. packages are each very small, and having these types in separate packages means that you have to add multiple imports just to send a histogram or a span. I think that it'd be more convenient if those types moved into the senders package.