[docs] Improve docstrings for middleware, skipclean (gorilla#375)

[feat] Add middleware support as discussed in gorilla#293 (gorilla#294)

* mux.Router now has a `Use` method that allows you to add middleware to request processing.

MatchErr is set to ErrNotFound if NotFoundHandler is used (gorilla#311)

[docs] README.md: add missing `.` (gorilla#292)

Support building URLs with non-http schemes. (gorilla#260)

* Move misplaced tests and fix comments.

* Support building URLs with non-http schemes.

- Capture first scheme configured for a route for use when building
  URLs.
- Add new Route.URLScheme method similar to URLHost and URLPath.
- Update Route.URLHost and Route.URL to use the captured scheme if
  present.

* Remove Route.URLScheme method.

* Remove UTF-8 BOM.

[bugfix] fail fast if regex is incorrectly specified using capturing …

…groups. (gorilla#218)

Merge pull request gorilla#199 from wirehead/minor-doc-tweek

Clarify how route variables work. (gorilla#151)

Merge pull request gorilla#158 from raninho/master

[tests] old_test: const DefaultRemoteAddr is unused.