Tracelog refactoring

Previously the initialization of the trace log was hard-coded in the `NancyContext`. This pull-request introduces the `IRequestTraceFactory` concept which provided a factory for setting up the request trace stuff. 

- Introduced `IRequestTrace` interface
- Renamed `RequesetTrace` to `DefaultRequestTrace`
- Introduced `RequestData` and `ResponseData`
- Introduced `IRequestTraceFactory` interface
- Implemented `DefaultRequestTraceFactory`
- Updated `DefaultNancyContextFactory` to use `IRequestTraceFactory`
- Added missing XML comments
- Added `StaticConfigurationContext` class to `Nancy.Testing`