dogwrap: decode process output as utf-8

[pbitty]

This fixes the issue where dogwrap will crash when a process' stdout/stderr contains non-ascii characters, in most cases utf-8.

I was getting this error:

$ python -m dogshell.wrap 'echo "\xE2\x98\xA0"'
Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/Library/Python/2.7/site-packages/dogshell/wrap.py", line 155, in <module>
    main()
  File "/Library/Python/2.7/site-packages/dogshell/wrap.py", line 139, in main
    event_body = u''.join(event_body)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 0: ordinal not in range(128)

With this change:

$ python -m dogshell.wrap 'echo "\xE2\x98\xA0"'

☠

[yannmh]

Thanks a lot @pbitty, I am merging it.