Add unit tests for `HasTraits.sync_traits`
Add further sync_trait tests
Improve docstring of `HasTraits.remove_trait`.
Add tests for `add_trait` and `remove_trait`.
Remove unused trait definitions in test.
Improved unit test for deleting a synchronized trait
Merge branch 'tst-improve-coverage-hastraits' of https://github.com/enthought/traits into tst-improve-coverage-hastraits
Fix typos.
Fix test for deleted sync trait.
Merge pull request #114 from enthought/tst-improve-coverage-hastraits

Unit tests for HasTraits class