Fix repository_tool.py issues reported.

Reported Issues:
1. Fix 'files_directory' variable name typo in comment.  Convert get_filepaths_in_directory() to a static method.

2. add_targets() declares 'absolute_list_of_targets' but only appends files to it.  Get rid of this variable.

3. The roleinfo dict of delegated roles was not fully set by load_repository().  Writing new metadata after it had been tried to access a non-existent 'partial_loaded' dict key.  Non-public function that decided whether a metadata file was partially loaded renamed to _metadata_is_partially_loaded(), and now returns a Boolean.

Minor edits to comments related to the fixes above.
Merge pull request #193 from vladimir-v-diaz/review_fixes

Fix repository_tool.py issues reported.
Merge branch 'develop' of https://github.com/theupdateframework/tuf into refactor_test_arbitrary_package_attacks