Renamed log to debug to avoid unnecessary overloading
Moved the images used by the apps to a common location.

This way apps can share images.
Fixed a race condition in loop_invariance_extern_calls test
Fixed race condition in a better way
Added a folder for a Halide tutorial.

Currently only has one lesson.

Also runs the lessons along with the unit tests, to make sure we
don't break them.
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.
Changes to CodeGen_C to allow result to compile & run properly

-- provide only a single definition of buffer_t, rather than two
different ones (ahem)
-- Added additional wrapper functions for a few more externs (ceil_f32,
floor_f32, etc). Note: I didn't attempt to exhaustively provide all of
them, so more boring work might be needed here
-- Added Intrinsic handling for the bitwise ops and reinterpret
-- Added void-casting to work around "unused variable" warnings in a
few cases

(There's some noise in the diff due to stripping of trailing
whitespace; if this is unacceptable then I'll resubmit)
Merge pull request #95 from steven-johnson/master

Changes to CodeGen_C to allow result to compile & run properly
merged with master and nick-sandbox
removed print statements
removed environment variable check in clamped vector load