Fixed string handling for Python 3

This change provides compatibility with Python 3 for recommended use cases. Requires Cython 0.19 or later.

Some tests in test_axographio.py are still failing in Python 3 (but not Python 2). Iterating over linearsequence and scaledarray objects currently fails in Python 3. This bug does not affect conversion of these objects to arrays using numpy.asarray or scipy.asarray, which is the recommended usage.

This change also explicity handles strings using ASCII encoding, so file paths containing non-ASCII characters are currently not supported in Python 3. Strangely, they nevertheless work in Python 2. Column names containing non-ASCII characters are rendered with hexadecimal escape codes in both Python 2 and 3.

axographio.pyx

@@ -1,3 +1,5 @@
+# cython: c_string_type=str, c_string_encoding=ascii
+
 import numpy as np
 cimport numpy as np