Refactor the library a bit

- Move test sub-package to webconnex/tester
- Move inject sub-package to webconnex/inject
- Merge other sub-packages into root package
- Add WriteValue() to ResponseWriter
- Update REST handler to use WriteValue()
- Display error from Context.Value()
- Remove Resource() from Router
- Rename Injector.Get Injector.Map
- Rename Injector.GetValue to Injector.MapValue
- Change Injector.Invoke() so it panics if arg not found