Issue #23 - add multipart/form-data usage example

Adding a working example of a handler which implements multipart/form-data handling using binding.
Putting `go` on a line after the backticks includes `go` in the code block. In fact, `go` here is meant to convey which language syntax highlighter should be used in the code block. See https://help.github.com/articles/github-flavored-markdown/