Added Python versions of base64 examples

These are transliterations of the existing C files.
They're not particularly Pythonic or performant, but they're a start.

Example of usage

```
$ echo '  YW55IGNhcm5hbCBwbGVhcw==' | PYTHONPATH=../build/opt/src/bindings/python/ python base64.py
inputsize=27
input=  YW55IGNhcm5hbCBwbGVhcw==
((((89L, 87L, 53L, 53L), (73L, 71L, 78L, 104L), (99L, 109L, 53L, 104L), (98L, 67L, 66L, 119L), (98L, 71L, 86L, 104L)), (99L, 'w', '=', '=')),)
$ echo '  YW55IGNhcm5hbCBwbGVhcw==' | PYTHONPATH=../build/opt/src/bindings/python/ python base64_sem1.py
inputsize=27
input=  YW55IGNhcm5hbCBwbGVhcw==
(97L, 110L, 121L, 32L, 99L, 97L, 114L, 110L, 97L, 108L, 32L, 112L, 108L, 101L, 97L, 115L)
$ echo '  YW55IGNhcm5hbCBwbGVhcw==' | PYTHONPATH=../build/opt/src/bindings/python/ python base64_sem2.py
inputsize=27
input=  YW55IGNhcm5hbCBwbGVhcw==
(97L, 110L, 121L, 32L, 99L, 97L, 114L, 110L, 97L, 108L, 32L, 112L, 108L, 101L, 97L, 115L)
```

examples/base64.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python2
+
+# Example parser: Base64, syntax only.
+#
+# Demonstrates how to construct a Hammer parser that recognizes valid Base64
+# sequences.
+#
+# Note that no semantic evaluation of the sequence is performed, i.e. the
+# byte sequence being represented is not returned, or determined. See
+# base64_sem1.py and base64_sem2.py for examples how to attach appropriate
+# semantic actions to the grammar.
+
+from __future__ import print_function
+
+import sys
+
+import hammer as h
+
+
+def init_parser():
+    # CORE
+    digit = h.ch_range(0x30, 0x39)
+    alpha = h.choice(h.ch_range(0x41, 0x5a), h.ch_range(0x61, 0x7a))
+
+    # AUX.
+    plus = h.ch('+')
+    slash = h.ch('/')
+    equals = h.ch('=')
+
+    bsfdig = h.choice(alpha, digit, plus, slash)
+    bsfdig_4bit = h.in_('AEIMQUYcgkosw048')
+    bsfdig_2bit = h.in_('AQgw')
+    base64_3 = h.repeat_n(bsfdig, 4)
+    base64_2 = h.sequence(bsfdig, bsfdig, bsfdig_4bit, equals)
+    base64_1 = h.sequence(bsfdig, bsfdig_2bit, equals, equals)
+    base64 = h.sequence(h.many(base64_3),
+                        h.optional(h.choice(base64_2, base64_1)))
+
+    return h.sequence(h.whitespace(base64), h.whitespace(h.end_p()))
+
+
+def main():
+    document = init_parser()
+
+    s = sys.stdin.read()
+    inputsize = len(s)
+    print('inputsize=%i' % inputsize, file=sys.stderr)
+    print('input=%s' % s, file=sys.stderr, end='')
+
+    result = document.parse(s)
+
+    if result:
+        #print('parsed=%i bytes', result.bit_length/8, file=sys.stderr)
+        print(result)
+
+
+if __name__ == '__main__':
+    import sys
+
+    main()