Support for Dropbox-API-Path-Root header

This PR allows users to set the Dropbox-API-Path-Root header globally for the client instantiation. This allows the Dropbox object to be scoped to a team namespace instead of an individual user's namespace.

Information on Dropbox's use of namespaces can be found here: https://developers.dropbox.com/dbx-team-files-guide#namespaces

Solves issue #207

Changes

compser.json

• Added phpunit support for versions 7 and 8
• Added ext-json to facilitate creating a json to pass the header data

Dropbox.php

• Added namespace_id to the config options
• Set the namespace id of the http client

DropboxGuzzleHttpClient.php

• Added a new protected property namespaceId of type string|null
• Added a setNamespaceId() method which accepts string or null as it's input and sets the protected property
• Updated send() to add a Dropbox-API-Path-Root header to the headers array if namespaceId is set