Prototype on removing targets from bins.
Minor edits.
Merge branch 'remove_target_from_bin' of https://github.com/dachshund/tuf into dachshund-remove_target_from_bin
Update comments and add missing docstrings in PR #216.

Update add_target_to_bin() and remove_target_from_bin().
Add missing test case for remove_target_from_bin().
Merge branch 'dachshund-remove_target_from_bin' into develop