Add `DnumberRestClient` for interacting with DNumber API

This PR adds the first implementation of `DnumberRestClient` for interacting with the DNumber REST API. Currently, the only implemented endpoint is `/v1/dnumbers/{id}`.

Please review: @ahoffmann @weizhong-wang

See merge request !3

Provide public access modifier for public functions

Also, fix some small PSR-2 formatting violations

@andrew-summers @weizhong-wang @npblair

See merge request !2

Fix some small PSR-2 formatting violations

Add FundingSource class to encapuslate funding source

This PR adds a `FundingSource` class to encapsulate the data returned by the the DoIT Number API key "fundingSource".

Please review: @ahoffmann @weizhong-wang

See merge request !1