Add test coverage using Coveralls

- Alter travis file to add test coverage.
- Add test coverage status badge to readme.
Update coverage badge to point to dev branch

[ci skip]
Merge pull request #11 from apel/add_coverage

Add test coverage using Coveralls
Update README.md

Change coverage badge to link to dev branch.
Limit coverage test to ssm and bin dirs
Make bin dir ref absolute for coverage
Tidy up travis file.

Remove path to bin as it doesn't work as not a Python package.
Merge pull request #12 from apel/alter_coverage

Limit code coverage test to ssm package
Add files in bin to tests

- Add __init__.py to bin so that those modules can be imported by tests.
- Add files in bin to coverage test.
Add skeleton test for bin/receiver

- Add skeleton test for bin/receiver so that the coverage stats become
  more accurate.
Add test_receiver to selective tests

- Add test_receiver to selective tests otherwise that unit test won't run
  and the coverage stats won't update for the modules in bin.
Add more tests for get_dns from receiver.py
Correct get_dns unit test
Merge pull request #14 from apel/bin-coverage

Add bin files coverage
Add code health badge to README.md
Add test_crypto to test run

- Change manual list of unit tests to automatic test discovery so that
  test_crypto runs so that we can fix it.
Add buffering to unit tests

- Add buffering to unit tests so that output is kept with the output of
  the relevant test.
Correct placement of buffer option
Update certificate and signed/encrypted messages

- Update certificate and signed/encrypted messages in test_cytpto as the
  old certificate was no longer valid.
- Add comments that show the commands used to generate the cert and
  messages.
Regenerate signed message with MIME text header