Fix bug report generation under Windows and OS X

2015-06-03 10:31:45,519 8212 14040 ERROR    nxdrive.wui.dialog 'ascii' codec can't decode byte 0xe9 in position 9234: ordinal not in range(128)
Traceback (most recent call last):
  File "nuxeo-drive-client\nxdrive\wui\dialog.py", line 461, in generate_report
  File "nuxeo-drive-client\nxdrive\manager.py", line 642, in generate_report
  File "nuxeo-drive-client\nxdrive\report.py", line 64, in generate
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe9 in position 9234: ordinal not in range(128)

2015-06-10 13:50:34,278 5608 4036 ERROR    nxdrive.wui.dialog 'ascii' codec can't decode byte 0x92 in position 109: ordinal not in range(128)
Traceback (most recent call last):
  File "nuxeo-drive-client\nxdrive\wui\dialog.py", line 461, in generate_report
  File "nuxeo-drive-client\nxdrive\manager.py", line 642, in generate_report
  File "nuxeo-drive-client\nxdrive\report.py", line 64, in generate
  File "nuxeo-drive-client\nxdrive\report.py", line 51, in _export_logs
UnicodeDecodeError: 'ascii' codec can't decode byte 0x92 in position 109: ordinal not in range(128)