Support running under python3, windows and OSX

[melissaebrooks]

The code was written to work with Python 2.7.9. Code changes will be required to run under python3. This issue covers that work.

[corinnebosley]

Following a preliminary recipe and build of this software for the Scientific Software Stack, the tests pass for Python 2.7 but not for Python 3.4.

These are the results of the tests:
https://circleci.com/gh/SciTools/conda-recipes-scitools/452?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

Here are some examples of the test failures for Python3.4:

File "/opt/conda/conda-bld/image-meta-tag_1478703759253/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehol/lib/python3.4/site-packages/ImageMetaTag/img_dict.py", line 333
    print 'IN: level: %s, before changes: %s, key "%s" in %s' \
                                                            ^
SyntaxError: Missing parentheses in call to 'print'

  File "/opt/conda/conda-bld/image-meta-tag_1478703759253/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehol/lib/python3.4/site-packages/ImageMetaTag/savefig.py", line 102
    print 'Image post-processing took: %s' %(str(datetime.now() - postproc_st))
                                         ^
SyntaxError: invalid syntax

  File "/opt/conda/conda-bld/image-meta-tag_1478703759253/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehol/lib/python3.4/site-packages/ImageMetaTag/webpage.py", line 255
    os.chmod(tmp_json_filepath, 0644)
                                   ^
SyntaxError: invalid token

compiling .pyc files...
  File "lib/python3.4/site-packages/ImageMetaTag/savefig.py", line 102
    print 'Image post-processing took: %s' %(str(datetime.now() - postproc_st))
                                         ^
SyntaxError: invalid syntax

  File "lib/python3.4/site-packages/ImageMetaTag/db.py", line 92
    print '%s database timeout writing to file "%s", %s s' \
                                                         ^
SyntaxError: Missing parentheses in call to 'print'

  File "lib/python3.4/site-packages/ImageMetaTag/webpage.py", line 255
    os.chmod(tmp_json_filepath, 0644)

[melissaebrooks]

#107 and #108: vn0.7.0 now supports python 3 and has been tested under python3.6.5.
It is available under conda and could be made available under pypi etc.