Enforce unicode json output for jsonutils.load[s]()

simplejson module applies some optimizations on ASCII-only unicode
strings which result in non-unicode json output. See details at [1]
and [2]. To make sure we always return consistent json output no
matter which json implementation is used, we should explicitly convert
the input for json.load[s]() to unicode.

If user wants to pass a file object of non UTF-8 encoding to
json.load[s](), she must also specify this encoding as an argument. To
support this scenario too, we've added 'encoding' argument to
jsonutils.load[s]() implementation.

Made all present JSON tests to run on both supported json library
implementations.

Added explicit dependency for simplejson to be able to test different
implementations in unit tests. Distributors still running Python 2.6
are recommended but not required to install simplejson.

Related-Bug: 1314129

[1]: https://code.djangoproject.com/ticket/11742
[2]: https://code.google.com/p/simplejson/issues/detail?id=40

Conflicts:
	test-requirements-py3.txt
	tests/unit/test_jsonutils.py

Change-Id: Ic815ca3df94c33edec9104172048b2cd94b92e3f
(cherry picked from commit 18f2bc1bf080b41a22c70842e7c127da21c63b8b)