Add Python 3 support

[myint]

This resolves the following problems under Python 3 while maintaining Python 2.7 support.

Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/myint/tmp/cmake_format/cmake_format/__main__.py", line 245, in <module>
    sys.exit(main())
  File "/Users/myint/tmp/cmake_format/cmake_format/__main__.py", line 184, in main
    for key, value in configuration.Configuration().as_dict().iteritems():
AttributeError: 'dict' object has no attribute 'iteritems'

Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/myint/tmp/cmake_format/cmake_format/__main__.py", line 245, in <module>
    sys.exit(main())
  File "/Users/myint/tmp/cmake_format/cmake_format/__main__.py", line 223, in main
    process_file(cfg, infile, outfile)
  File "/Users/myint/tmp/cmake_format/cmake_format/__main__.py", line 38, in process_file
    pretty_printer.print_node(fst)
  File "/Users/myint/tmp/cmake_format/cmake_format/formatter.py", line 594, in print_node
    self.print_block(node)
  File "/Users/myint/tmp/cmake_format/cmake_format/formatter.py", line 453, in print_block
    self.print_node(child)
  File "/Users/myint/tmp/cmake_format/cmake_format/formatter.py", line 600, in print_node
    self.print_statement(node)
  File "/Users/myint/tmp/cmake_format/cmake_format/formatter.py", line 573, in print_statement
    self.outfile.write(self.get_indent())
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/tempfile.py", line 483, in func_wrapper
    return func(*args, **kwargs)
TypeError: a bytes-like object is required, not 'str'

Travis CI results for Python 2.7 and 3.6.

[cheshirekow]

FYI: Looks like the io module exists in python 2.7 as well..

[DanielChabrowski]

But StringIO doesn't in 3.0+.

The StringIO and cStringIO modules are gone. Instead, import the io module and use io.StringIO or io.BytesIO for text and data respectively.

edit:
Or do you mean that you can import StringIO from io in python 2? Then dunno.