Reorder load_signing_key() calls.

The Targets section of the README separated import_rsa_privatekey_from_file() from load_signing_key().  To improve readability, reorder them so that load_signing_key() calls come immediately after the key import method calls.
Add links and Python prompts.
Minor edits to repository_tool.py.
Initial implementation of custom FILEINFO
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