Add support for file uploads in MultipartForm

The variable being bound needs to have a field of type
*multipart.FileHeader (or []*multipart.FileHeader). Responsibility for
doing anything with the upload lives further down the middleware chain.
json: do not treat an empty body as an error
Handle empty payload test case
Add unsigned int support when binding forms
Fix int64 parsing on 32bit platforms
Merge pull request #8 from 3XX0/reflect-fix

Form binding - reflect fixes
Merge pull request #7 from etix/master

json: do not treat an empty body as an error
Merge branch 'master' of https://github.com/p-lewis/binding into p-lewis-master

Conflicts:
	binding.go
	binding_test.go
Add MultipartForm documentation to README
Merge pull request #11 from p-lewis/master

Add MultipartForm documentation to README