Add AST Visitors.

Add visitor interfaces for the Expression and Statement family of classes.
	src/Compiler/AST/IExpressionVisitor.h
	src/Compiler/AST/IStatementVisitor.h

Implement the Accept method on all Expression and Statement classes
	src/Compiler/AST/Expression*
	src/Compiler/AST/Statement*
Move Platform Source Files.

IPlatform and its implementations belong in the Platform namespace, not CodeGen.
	src/Compiler/CodeGen/IPlatform.h -> src/Compiler/Platform/IPlatform.h
	src/Compiler/CodeGen/Platform_i386_OSX.h -> src/Compiler/Platform/Platform_i386_OSX.h
	src/Compiler/CodeGen/Platform_i386_OSX.cpp -> src/Compiler/Platform/Platform_i386_OSX.cpp

Fortunately, the fallout is quite minimal...
	project/Mac OS X/Compiler.xcodeproj/project.pbxproj
	src/code_gen.cpp