Rework and add new rules for simplifying ramp/x and ramp%x

Fixes issue #458
Fix infinite loop found with fuzz_simplify

ramp(cast(intimm), ...) was treated as a simple_const
Add additional ctors to Param.h

Allow optional specification of initial-value (or initial-value, min,
max) via ctor arguments to scalar Param<>.
Fix CMake for OSX

Fix glitches that prevent the CMake build from working on OSX:
— Must explicitly set -target flag for clang for -m64 flag to work (as
is done in Makefile)
— Remove unnecessary -ltinfo directive (doesn’t exist on OSX)
Add instructions for building Halide with cake
Merge pull request #496 from halide/ramp_div_mod_simplify_rules

Rework and add new rules for simplifying ramp/x and ramp%x
Merge pull request #499 from halide/param_ctor

Add additional ctors to Param.h
Update libpng version used by script.
Merge branch 'master' of github.com:halide/Halide
Merge branch 'master' of https://github.com/halide/Halide into typed-funcs
Fix makefile for predownloaded llvms.
Query llvm for system libs to use on os x and linux
cmake is the devil
Merge pull request #500 from halide/cmake

Fix CMake for OSX
Windows testing script tweak
Merge branch 'master' of https://github.com/halide/Halide into typed-funcs