[Python2] Trying to export a non-unicode string causes an error

[imaduro]

Exporting a parsed model that contains non-unicode strings causes the following error on Python2

>>> my_string = "value + value2"
>>> parsed = example_meta.model_from_str(my_string)
>>> from textx.export import model_export
>>> model_export(parsed, "my_file.dot")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/textx/export.py", line 163, in model_export
    model_export_to_file(f, model, repo)
  File "/usr/local/lib/python2.7/site-packages/textx/export.py", line 267, in model_export_to_file
    _export(model)
  File "/usr/local/lib/python2.7/site-packages/textx/export.py", line 222, in _export
    _export(list_obj)
  File "/usr/local/lib/python2.7/site-packages/textx/export.py", line 195, in _export
    for attr_name, attr in obj_cls._tx_attrs.items():
AttributeError: type object 'str' has no attribute '_tx_attrs'

[igordejanovic]

Related to #99
We should have a better error report if a user pass a non-unicode object to *_from_str.

[igordejanovic]

As of #120 textX strictly requires unicode.