Move the Plug.Router.build_host/1 method to Plug.Router.Utils

I move the above-mentioned method to `Plug.Router.Utils` and renamed it to the
more accurate `build_host_match/1`; I also added some doctests.

I also "incidentally" added some explanatory comments to the
`Plug.Router.forward/2` macro.
Merge pull request #138 from whatyouhide/tiny-cleanups

Move the Plug.Router.build_host_match/1 method to Plug.Router.Utils