Python3 compatibility fixes

[IlyaSemenov]

The latest version didn't work in Python 3.5 due to JSON loading errors:

ERROR: test_status_handled (tests.test_webpack_manifest.TestBundles)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/semenov/work/github/patches/python-webpack-manifest/tests/test_webpack_manifest.py", line 69, in test_status_handled
    static_url='/static',
  File "/Users/semenov/work/github/patches/python-webpack-manifest/webpack_manifest/webpack_manifest.py", line 108, in load
    manifest = build(path, static_url, debug, timeout, read_retry)
  File "/Users/semenov/work/github/patches/python-webpack-manifest/webpack_manifest/webpack_manifest.py", line 119, in build
    data = read(path, read_retry)
  File "/Users/semenov/work/github/patches/python-webpack-manifest/webpack_manifest/webpack_manifest.py", line 198, in read
    return json.loads(content)
  File "/Users/semenov/.pyenv/versions/3.5.2/lib/python3.5/json/__init__.py", line 312, in loads
    s.__class__.__name__))
TypeError: the JSON object must be str, not 'bytes'

Also, all the exception tests were broken in Python3 since Exception.message attribute was removed.

The PR fixes both problems. Tested in 2.7.12 and 3.5.2.

[markfinger]

Cool. Thanks, @IlyaSemenov!

I'll merge your changes and push a new release to PyPI this weekend.

[markfinger]

Thanks again, @IlyaSemenov. I've published your changes as 1.1.0