ERR: Reduced requirements on preprocessor for export macro due to limitations of Mac preprocessor.  The preprocessor can be fixed by using -no-cpp-precomp, but we don't want to require that option for every source file that includes our headers.

Removed Exception_Abort because there is no windows version.  Also made ExitValue consistent with windows when a signal kills the process.

ENH: Added documentation to interface.  Finished process exit code interpretation implementation prototype.

ERR: Using GetCurrentProcessId instead of _getpid so we don't need to include the system process.h header.  Also creating pipe threads with 1K stacks to save memory.

ENH: EXECUTABLE_OUTPUT_PATH is now always set to get around cmake 1.6.7 dependency problems.  Moved default header directory inside the build tree instead of up one level.  User projects can now set the header directory.