Exception when building xpath query #46

I managed to trigger this when downloading workbooks from alpo.

This was done against the HEAD of a local copy of #45

`Traceback (most recent call last):
File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/xml/etree/ElementPath.py", line 263, in iterfind
selector = _cache[cache_key]
KeyError: (".//metadata-record[@Class='column'][local-name='[Today's Date]']", None)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "runner.py", line 30, in
fields = wb_model.datasources[0].fields
File "/Users/tdoyle/Documents/py/document-api-python/tableaudocumentapi/datasource.py", line 135, in fields
self._fields = self._get_all_fields()
File "/Users/tdoyle/Documents/py/document-api-python/tableaudocumentapi/datasource.py", line 141, in _get_all_fields
return collections.OrderedDict([(k, v) for k, v in column_objects])
File "/Users/tdoyle/Documents/py/document-api-python/tableaudocumentapi/datasource.py", line 141, in
return collections.OrderedDict([(k, v) for k, v in column_objects])
File "/Users/tdoyle/Documents/py/document-api-python/tableaudocumentapi/datasource.py", line 140, in
for xml in self._datasourceTree.findall('.//column'))
File "/Users/tdoyle/Documents/py/document-api-python/tableaudocumentapi/datasource.py", line 17, in _mapping_from_xml
metadata_record = root_xml.find(xpath)
File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/xml/etree/ElementTree.py", line 649, in find
return self._root.find(path, namespaces)
File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/xml/etree/ElementPath.py", line 298, in find
return next(iterfind(elem, path, namespaces), None)
File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/xml/etree/ElementPath.py", line 277, in iterfind
selector.append(ops[token[0]](next, token))
File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/xml/etree/ElementPath.py", line 233, in prepare_predicate
raise SyntaxError("invalid predicate")
SyntaxError: invalid predicate`