Prototype on removing targets from bins.
Minor edits.
Fix comment / typo in unsafe_download() test condition
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
Fix expected format of the 'method_name' argument.
Merge pull request #228 from vladimir-v-diaz/develop

Review, update, document, and test #216.