Allow backend to be passed in to sendfile()

[kaedroho]

This pull request adds a kwarg called backend to sendfile(). It allows a different backend to be used for a particular view. It's also useful for third-party apps that want to use sendfile but provide a fallback if their users haven't set SENDFILE_BACKEND.

Example:

    from sendfile import sendfile
    from sendfile.backends.simple import sendfile as simple_sendfile

    def myview(request):
        ...

        return sendfile(..., backend=simple_sendfile)

[johnsensible]

Interesting. Curious as to use case though? The library is just intended to paper over differences between sendfile/nginx/xsendfile etc. Seems odd that you might want a different backend for one particular application?