ENH: Added GetExceptionString method to provide an error description when GetState returns Exception.

ENH: Added SetPipeShared method to allow stdout and stderr pipes to be shared with the parent process.

ENH: Added SetPipeFile method to allow the process pipeline stdin, stdout, and stderr to be redirected from/to files.

ENH: Removed pipe selection argument from WaitForData method in kwsysProcess.  This greatly simplifies its use.

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.

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