Fix for Issue 72: Markdown: 500 for YAML metadata

browse: return forbidden (403) only when it is a permission error.

headers: Initial test for Headers and change of Rule.Url to Rule.Path

basicauth: Should write an unauthorized code to response. (Actually only returns it)

Internal middleware: added tests

Also fixed bug for markdown files that don't contain front matter

Updated ServeHTTP comment to indicate that it is 
setting headers and not adding them to existing values.

redirect: initial test for Redirect

Removed body check and replaced urlPrinter with a inlined Next handler

Ensures metadata.Variables is made

Markdown: support for templates and metadata

Close #64 (Make it possible to remove headers)

Adding "internal" middleware

- Detect too many internal redirects - return 500 in this case

- redirect internally regardless of proxy status code
- support multiple internal redirects