making improvements to the consistency and functionality of the shell code

[aaronbog]

shell/utils.py:

• using shlex to split the commands, this will fix errors where the path contains spaces (fixes issue PerfWrapper does not handle spaces in paths #23 and issue when space in name package #9)

shell/shell.py:

1. rewritten shell_out function

• using true to determine the value of the success return value
• refactor such that all commands can use the same Popen structure for consistency
• output_is_log=True now takes into account the timeout variable (this is the use of the multiprocessing package)
• output_is_log=True now takes into account the ignore_ret_codes
• made allowed exit codes add their stderr to the output instead of overwriting it
• added split_arguments=False as an argument so pipe_shell_out can be simplified

2. rewritten pipe_shell_out function

• rewrote the implementation using shell_out and the new split_arguments argument