enables streaming stderr

[sbromberger]

Closes #28

[rogerpearce]

There's an issue. If the process exits before all of the stderr has been read, then it gets lost. Looks like it's missing a double check on reading remaining lines in the pipes after process closes. Example:

In [13]: mu.welcome()
======================================
 YY    YY     GGGGGG      MM     MM
Out[13]: 0

[rogerpearce]

It is still not streaming. Add some sleeps between cerr output and you will see, like:

comm.welcome(std::cerr);
 // Sleep for 2 seconds
 std::this_thread::sleep_for(std::chrono::seconds(2));
 comm.welcome(std::cerr);
 // Sleep for 2 seconds
 std::this_thread::sleep_for(std::chrono::seconds(2));
  comm.welcome(std::cerr);

The problem is that select.select ONLY works with file descriptors (fds) so that is all immediately ending.

[rogerpearce]

Another way to simplify if you wanted to is to only allow streaming form stderr and wait to even read stdout until after the process completes. Any future progress type stuff could be streamed to stdout.