HTTP file server

Initial implementation for serving static content from RxNetty based on https://github.com/netty/netty/blob/master/example/src/main/java/io/netty/example/http/file/HttpStaticFileServerHandler.java.  

Additional features include,
1.  Support for single and multiple file sources
2.  zero-copy serving of files

But the above example still has a 1-1 relationship between path and handler.  The functionality I've implemented would be more along the lines of 

```java
router.withRoute("/b", ImmutableList.of(new FileSystemBasedRouter("/var/files"), new ClassPathBaseRouter(), new RemoteUrlBasedRouter("http://someotherfileserver"));
```
The expected behavior would be that for any uri starting with "/b" we'd first look for the file in the folder /var/files.  If not found then look for the file in the class path.  If still not found then look for the file on a remote server.