Support different backends

[michaelneu]

We could support Microsoft Access and eventually Microsoft Word as well by creating a context-aware RestWebController, which internally uses e.g. the WorkbookWebController if the macro's run in Microsoft Excel.

The backends should behave like the WorkbookWebController currently does:

• return a JsonObject or JsonArray
• resolve relationships using fk_<table> keys
• automatically create REST endpoints based on the backend's structure (e.g. worksheets in Excel, database tables in Access)

The build script needs to be adjusted for that as well. Maybe we can use yeoman or a project file (e.g. webxcel.json) specifying the build process.