Adopt repository structure

tests/bookshop.http

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+### Service Document
+GET http://localhost:4004/browse
+
+### Service $metadata document
+GET http://localhost:4004/browse/$metadata
+
+### Browsing Books
+GET http://localhost:4004/browse/Books?
+# &$select=title,author
+# &$expand=currency
+# &sap-language=de
+
+### Browsing Authors
+GET http://localhost:4004/admin/Authors?
+# &$select=name,dateOfBirth,placeOfBirth
+# &$expand=books($select=title;$expand=currency)
+# &$filter=ID eq 101
+# &sap-language=de

tests/orders.http

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+
+### List Books with their current stocks
+GET http://localhost:4004/admin/Books?$select=ID,stock
+
+### List all Orders
+GET http://localhost:4004/admin/Orders?
+&$expand=Items
+
+### Submit Orders
+POST http://localhost:4004/browse/Orders
+Content-Type: application/json
+
+{ "OrderNo":"2019-09...", "Items":[
+    { "book_ID":201, "amount":5 },
+    { "book_ID":207, "amount":3 }
+]}
+
+# Sending this three times should result in a 409: 5 exceeds stock for book #201