Unicode strings with non-ASCII characters cause UnicodeEncodeError

[obi1kenobi]

Repro: use the testcase in #128 and add '\u2603' (unicode snowman) to the end of the unicode argument. Observe the following output:

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ERROR: testUnicode (tests.test_ogm.OGMUnicodeTestCase)
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Traceback (most recent call last):
  File "/Users/predrag/Code/pyorient/tests/test_ogm.py", line 327, in testUnicode
    g.unicode.create(name=name, value=value)
  File "/Users/predrag/Code/pyorient/pyorient/ogm/broker.py", line 53, in create
    return self.g.create_vertex(self.element_cls, **kwargs)
  File "/Users/predrag/Code/pyorient/pyorient/ogm/graph.py", line 272, in create_vertex
    for k,v in db_props.items()))
  File "/Users/predrag/Code/pyorient/pyorient/ogm/graph.py", line 272, in <genexpr>
    for k,v in db_props.items()))
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2603' in position 0: ordinal not in range(128)

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