Contributions: make user Func names unique; allow passing Image as Func

Hi

I have been enjoying Halide for a few weeks now and would like to offer a couple of small contributions to the project.

1. Solve the issue of programmer-specified function names not always being unique by tracking names used and adding unique counter.
2. Allow Image to be passed to C++ code that expects a Func parameter.

There is a trivial associated test program.

Regards
Len Hamey
Macquarie University               len.hamey@mq.edu.au