Added Analytics.js Library

#### Segment.io's Analytics.js

I've added Segment.io's Analytics.js library. They originate from their GitHub repository [here](https://github.com/segmentio/analytics.js). The latest version is version 1.3.29. As the library was configured with a `package.json` and was ready for NPM but wasn't actually published I published the latest version myself to NPM. The library is configured with auto-update. I'll continuously publish NPM updates - so long as Segment.io don't start releasing them themselves - every time a new release is made on their GitHub repository.

Proof of popularity can be seen at the above GitHub Repo.