Add test condition for repository_tool.Target().add_target() custom data.
Verify that generated Targets metadata can contain custom data.
Regenerate '/tests/repository_data/' to include example of custom data.
Add test condition for custom data in test_updater.py.

The updater client may reference target information (including 'custom') by accessing the dictionary returned by updater.py.

Example:
target = updater.target('LICENSE.txt')
target['filepath']
target['fileinfo']['length']
target['fileinfo']['hashes']
target['fileinfo']['custom']
Update README.md

Add example of custom data included with repository.targets.add_target()
Update README.md

Add example for accessing fileinfo and custom data of target objects.
Review and update docstrings and README.
Merge pull request #230 from vladimir-v-diaz/develop

Implementation of custom FILEINFO and documentation changes to repository_tool.py README
Update README.md

Fix target path variable name in custom data example.
Update README.md

Add instructions for contributors and minor edits to installation section.
Update README.md

Remove extra whitespace.
Update README.md

Add extension to requirements file.
Merge branch 'develop' of https://github.com/vladimir-v-diaz/tuf into develop
Merge branch 'SantiagoTorres-developer-tools' of https://github.com/vladimir-v-diaz/tuf into develop
Merge branch 'develop' of https://github.com/theupdateframework/tuf into SantiagoTorres-developer-tools
Merge custom info changes, logger messages, and minor edits.