Ignore BOM when parsing tags.

Patch for when there is a BOM at beginning of the bagit.txt being parsed. BOM gets included in the dict key for BagIt-Version and causes an error.
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "bagit.py", line 158, in __init__
    self._open()
  File "bagit.py", line 177, in _open
    raise BagError("Missing required tag in bagit.txt: %s" % e)
bagit.BagError: Missing required tag in bagit.txt: 'BagIt-Version'
```