Refactored modules (configuration.py, __init__.py, updater.py, README.md) in tuf.interposition package and added unit tests for tuf.interposition.updater.py

1. A test file named tuf.tests.test_interpose_updater.py, testing tuf.interposition.updater. Wrote test cases for all the methods in this module.
2. Added three exception classes in tuf.__init__.py
3. Added proper introduction for high-level updater i.e. tuf.interposition and explanation for each step taken to interpose in tuf.interposition.README.md
4. Removed a separate Logger and exception file for interposition named as tuf.interposition.utility and added the general logger used for TUF in interposition files.
5. In tuf.interposition.__init__.py and tuf.interposition.configuration.py, added the general logger and gave new names to the exception errors. Also, added document strings to this explaining lines of code.
6.  Added an introduction to tuf.interposition.updater.py explaining the purpose of the document and description regarding the requirements of high-level updater.
7. Added an introduction containing purpose, arguments, exceptions, side effects and returns for every method in tuf.interposition.updater.py.
8. Added comments for every line of code in tuf.interposition.updater.py. 
9. Fixed a bug regarding exception handling in tuf.interposition.updater.py.
10. In tuf.interposition.updater.py, the UpdaterController.get() method was dependent on hostname. But it should be dependent on network location (hostname+ port). So changed the code here.

TODO - Add test cases for tuf.interposition.configuration.py
Add features of python 3