Python 3 support

[jbarlow-mcafee]

This PR adds support for running under Python 3.4+, while retaining the ability to run under Python 2.7.

This PR is dependent upon releases of the OpenDXL Python client and bootstrap packages which support Python 3.4+, which is being added in opendxl/opendxl-client-python#26 and opendxl/opendxl-bootstrap-python#3, respectively. The Travis CI configuration temporarily depends upon branches of the OpenDXL Python client and bootstrap packages which support Python 3. The reference to the branches can be removed when Python 3-capable client and bootstrap releases is available.

Notable changes include:

• Package could be installed/used with Python 3.4+. Changes have been tested up to Python 3.6.4.
• A universal wheel package (single package which can be installed on either Python 2.7.9+ or Python 3.4.0+) will be built instead of a version-specific package.
• Package zip files will no longer include .egg files. These would need to be built uniquely per Python version - e.g., 2.7 vs. 3.4, 3.5, and 3.6.
• CI testing in Travis now includes running pylint against project sources and samples to help ensure version compatibility. The Travis CI build matrix now includes Python 2.7, 3.4, 3.5, and 3.6.
• Packaging has been updated for use with functionality in bootstrap 0.1.3 and later.
• A simple unit test which exercises the maxmind sample was added.