Py3 Compat

[lmignon]

This PR makes the module compatible with Python 3 while remaining compatible with Python 2.7.

The first step in the migration process was the implementation of a continuous integration process based on travis and the refactoring of the tests. (see #10 for the motivations)

In a second step, the code was modified syntactically to be both Python 2 and Python 3 compliant.

The httplib2 library was then replaced by python-request (http://docs.python-requests.org/en/master/). It natively supports Python 2 and Python 3, manages sessions better (perf are improved), has no problem with large files and is very well maintained and documented. This library seems to be a de facto standard.

Finally, the code is now flake8.

These changes constitute a major evolution of the library. Even if all the unit tests allow us to believe that no regression was introduced by this refactoring, these changes must be validated by an intensive use of the library.

[x2764tech]

What would it take to get this into an official release?

[lmignon]

@x2764tech We have to ask on the chemistry mailing list. dev@chemistry.apache.org
Do you have some feedback to provide on this new version? I use it in many projects without any problems but only the BrowserBinding part. I would be interested to know if other people have had the opportunity to test this new version.

[x2764tech]

I'm planning on using the AtomPub part, to integrate with JupyterHub, so I'll feed back any issues I find there.
Using Session for the requests makes a lot of things a lot simpler too.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (trunk@e11e66b). Click here to learn what that means.
The diff coverage is 75.13%.

@@           Coverage Diff           @@
##             trunk     #12   +/-   ##
=======================================
  Coverage         ?   67.4%           
=======================================
  Files            ?      11           
  Lines            ?    3215           
  Branches         ?     471           
=======================================
  Hits             ?    2167           
  Misses           ?     861           
  Partials         ?     187

Impacted Files	Coverage Δ
src/cmislib/atompub/binding.py	65.16% <ø> (ø)
src/cmislib/exceptions.py	100% <100%> (ø)
src/cmislib/__init__.py	100% <100%> (ø)
src/cmislib/messages.py	100% <100%> (ø)
src/cmislib/cmis_services.py	58.73% <57.4%> (ø)
src/cmislib/domain.py	59.52% <66.66%> (ø)
src/cmislib/util.py	71.42% <72.09%> (ø)
src/cmislib/browser/binding.py	71.42% <78.57%> (ø)
src/cmislib/model.py	90.9% <88.88%> (ø)

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