ES6 cleanup and use MongoDB as a backing store

[bensalilijames]

Description

Similar to other SS cleanup projects: ES6 used and general tidyup. This also uses MongoDB as a backing store rather than a file system, which allows SuperScript to scale (on AWS Lambda etc).

Motivation and Context

Have a common ES6 basis across all SS projects for better readability and code quality in general, and allow SS to scale by having central fact database on a server, rather than distributed filesystem folders of facts (which would have to be sync'ed somehow).

How Has This Been Tested?

Run all existing unit tests with success.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Only breaking change is that sfacts.expand() is no longer exposed to the user. It was previously aliasing sfacts.create(), so this should be used instead.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Changes Unknown when pulling 103dbd6 on benhjames:master into * on superscriptjs:master*.