Python 2/3 compatibility

Updated data2passcal to work with both Python 2 and 3.

• Resources used: Python-Future project and Futurize tool.
• Changes tested using unittest module on individual test methods(tests.test_data2passcal)
• Changes tested against real seismic dataset (AB.2002 - 2012)
• Added some unit tests to ensure basic functionality of data2passcal
• Updated list of platform specific dependencies to be installed when installing data2passcal in dev mode (see setup.py)
• Installed and tested data2passcal against Python2.7 and Python3.6 using tox
• Formatted Python code to conform to the PEP8 style guide (exception: E722, E712, E501)

CONTRIBUTING.rst

@@ -76,7 +76,8 @@ Ready to contribute? Here's how to set up `data2passcal` for local development.
 5. When you're done making changes, check that your changes pass the
    tests::
 
-    $ python setup.py test
+    $ python -m unittest (or python -m unittest tests.test_data2passcal under Python2)
+
 6. Commit your changes and push your branch to GitHub::
 
     $ git add .
@@ -111,4 +112,3 @@ Then run::
 
 $ git push
 $ git push --tags
-