Update preprocess.py to support Python 3

[mattbierner]

Print statements to print function calls.
Use items instead of iteritems.

Tested on Python 2.7 and Python 3.5

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Running preprocess.py under Python 3.3+ fixes #29

For the following text file:

Test 😀!

Here's the (incorrect) Python 2.7 json output:

{"idx_to_token": {"1": "T", "2": "e", "3": "s", "4": "t", "5": " ", "6": "\ud83d", "7": "\ude00", "8": "!", "9": "\n"}, "token_to_idx": {"!": 8, " ": 5, "e": 2, "\ude00": 7, "\n": 9, "s": 3, "T": 1, "\ud83d": 6, "t": 4}}

And the correct Python3.3 json:

{"idx_to_token": {"1": "T", "2": "e", "3": "s", "4": "t", "5": " ", "6": "\ud83d\ude00", "7": "!", "8": "\n"}, "token_to_idx": {"t": 4, "s": 3, " ": 5, "\ud83d\ude00": 6, "!": 7, "e": 2, "\n": 8, "T": 1}}

[gokceneraslan]

Let's please consider #12 to get rid of the Python dependency, rather than enhancing it.

[ChrisCummins]

Python 3 support merged in 37a31bb, using six package for iterators, and removing a broken dep.