Add composer settings.

Ensure that composer will install development version of dts/ebay-sdk.
Add base service class.
Add service constants.
Merge branch 'consts' into develop
Add configuration properties to the service.
Merge branch 'config' into develop
Service class is passed HttpClient instance via ctor.
Merge branch 'http-client' into develop
Add produciton and sandbox urls.
Merge branch 'sandbox' into develop
Generate correct eBay http headers.
Merge branch 'http-headers' into develop
Move test classes into mock directory and add test namespace.
Merge branch 'test-namespace' into develop
Make tests pass due to changes in ebay-sdk.
Merge branch 'fixes' into develop
Update composer details.
Merge branch 'packagist' into develop
Update license in composer detail.
Merge branch 'packagist' into develop