Added an early-out to skip-stages to speed it up
Bugfix for previous commit
Added a note in the runtime header about weak linking.
Added an app that does modular compilation of a pipeline.
Method name fix in comment.
Added ability to add runtime arguments to assertion failure messages.

halide_error now has a printf-like type signature to support this.
Added PNaCl backend and made the HelloNaCl app use it.

It turns out to just be a matter of generating a bitcode file with the
right target triple and data layout.
Don't vectorize assert statements (now that they take varargs)
pnacl doesn't seem to use the llvm math intrinsics
Made nacl_demos work with pnacl

Also added missing files from HelloNaCl, and corrected misconception
about needing --pnacl-disable-abi-check
Fixed nacl_demos Makefile to include pnacl stuff in the zip
Example app for bilinear upscaling of images
Updated reaction diffusion 2 to be slightly more interesting
Better error messages in nacl demos
Add bicubic interpolation
Updated type on overrides of halide_error
Add command line handling and support for downscaling
On second thought, variadic functions in the runtime are lame.

Reverted halide_error to non-variadic, and added an internal vsnprintf to catch
and stringify the variadic args.
Simplified the math in reaction diffusion demo.
More interesting initial and boundary conditions for reaction diffusion 2