Switched off automatic pipeline hookup for static content

To enable it users now need to do StaticContent.Enable(pipelines)

Updated tests accordingly.
Added IStaticContentProvider and implementations
Added to internal configuration
Hooked into NancyEngine
Added failing tests for conditional requests
Added automatic etag/last modified 304 responses for static content
Added content type null checks
Fix for WCF and not-modified static content

WCF forces us to specify a content type, even though we're
sending no body, although I can't say it's a huge surprise
to see WCF breaking HTTP.. *sigh*
Merge pull request #982 from grumpydev/FirstClassStaticContent

First class static content