Added Billing API methods:
* POST: purchases
* GET: purchases/{id}
* GET: users/current/products
* POST: purchases/{id}/refund

Added configuration for billing api endpoint
Work in progress support for GET:search
Support for GET:search
Merge branch 'features/search' of github.com:basespace/basespace-csharp-sdk into feature/search-master
Merge pull request #17 from basespace/feature/search-master

Feature/search master
Merge branch 'billingchanges' of github.com:basespace/basespace-csharp-sdk into feature/billingchanges-master
Synced with the upstream repo