Add Func names to assertion failures in Function.cpp, to make diagnosing common mistakes easier
Add inline wrappers for sqrt_f32, abs_f32 in CodeGen_C.cpp
Type codes were wrong in the docs.
Merge pull request #94 from steven-johnson/master

Add Func names to assertion failures in Function.cpp
Fixed comment in image_io.h
Added a second lesson to tutorial/

And updated the first one slightly.
Merge branch 'master' into nick-sandbox