Disable "universal newlines" to avoid unwanted changes

Python 3 is using "universal newlines" by default when reading text files, which means that foreign line terminators are silently discarded and are never seen by the program; to disable this behavior, newline='' (empty string) must be added to open(), see http://docs.python.org/3/library/functions.html#open

Test cases:
```
python -c "print('teh')," >test
python -c "print('teh\r')," >test-cr
python -c "print('teh\n')," >test-lf
python -c "print('teh\r\n')," >test-crlf
python -c "print('teh\n\r')," >test-lfcr
for f in test*; do echo === $f ===; hexdump -C $f; ~/Programmazione/codespell/codespell.py -w $f; hexdump -C $f; done
```
Output when running on Linux before this patch (note how 0d is changed to 0a except in test-crlf where it disappears):
```
=== test ===
00000000  74 65 68 0a                                       |teh.|
00000004
FIXED: test
00000000  74 68 65 0a                                       |the.|
00000004
=== test-cr ===
00000000  74 65 68 0d                                       |teh.|
00000004
FIXED: test-cr
00000000  74 68 65 0a                                       |the.|
00000004
=== test-crlf ===
00000000  74 65 68 0d 0a                                    |teh..|
00000005
FIXED: test-crlf
00000000  74 68 65 0a                                       |the.|
00000004
=== test-lf ===
00000000  74 65 68 0a                                       |teh.|
00000004
FIXED: test-lf
00000000  74 68 65 0a                                       |the.|
00000004
=== test-lfcr ===
00000000  74 65 68 0a 0d                                    |teh..|
00000005
FIXED: test-lfcr
00000000  74 68 65 0a 0a                                    |the..|
00000005
```
Output when running on Linux after this patch (0d is preserved):
```
=== test ===
00000000  74 65 68 0a                                       |teh.|
00000004
FIXED: test
00000000  74 68 65 0a                                       |the.|
00000004
=== test-cr ===
00000000  74 65 68 0d                                       |teh.|
00000004
FIXED: test-cr
00000000  74 68 65 0d                                       |the.|
00000004
=== test-crlf ===
00000000  74 65 68 0d 0a                                    |teh..|
00000005
FIXED: test-crlf
00000000  74 68 65 0d 0a                                    |the..|
00000005
=== test-lf ===
00000000  74 65 68 0a                                       |teh.|
00000004
FIXED: test-lf
00000000  74 68 65 0a                                       |the.|
00000004
=== test-lfcr ===
00000000  74 65 68 0a 0d                                    |teh..|
00000005
FIXED: test-lfcr
00000000  74 68 65 0a 0d                                    |the..|
00000005
```