Implement Segments API

[randyzwitch]

Supersedes #11

Segments API covered here:
https://github.com/AdobeDocs/analytics-2.0-apis/blob/master/segments-guide.md
https://github.com/AdobeDocs/analytics-2.0-apis/blob/master/segments.md

It appears that Adobe really doesn't want people to programmatically create segments, instead suggesting users defer to using the UI. This looks like something to consider when building out this library; deleting segments probably pretty simple and useful for bulk removing of segments. Creating and Updating segments possibly a bit less so