I did a couple of improvments in Valum

I really see much potential in this web framework!

Here's a summary of the changes
- I made `View` abstract instead of an interface as it can conviniently carry its variables.
- Ctpl can fetch a tree from a path, so I have added a `from_path` function in its view engine.
- first route matching handles the `Request`
- Soup Message is completed by the router instead of being completed in the callback (this avoids unterminating response..)
- Ctpl render any view errors instead of showing a blank page
- I added more getter/setter in Request to access headers and body
- Headers should be handled as Soup MessageHeaders instead of a HashMap<string, string> as it is more flexible and lightweight at implementation level

I really think this framework like a micro-framework that provide basic functionnalities and elegant callback for routing.