Added soft wrappers for the add_verification_key method
The add verification key method now prohibits the ability to have more
than one key in the Project role, however, delegations do support
having more.

This is the suggested behavior as of this version, removing these two
wapper functions would prevent to have this limitation but would require
the extension of the write_project_configuration function
Updated the load project method to include the Targets inherittance
The load_project contained statements that called the previously
included "targets" instance, since we now inherit Targets, we don't
require them, the statements were updated the following way:

self._targets

to a plain:

self
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 github.com:theupdateframework/tuf into developer-tools
The _check_if_partial_loaded method was renamed
The repository tool now provides a method called
"_metadata_is_partially_loaded" to provide the same functionality, the
return type changed so the flow of this module had to adapt to the new
function:

_check_if_partial_loaded() -> updated the roleinfo data as an effect

_metadata_is_partially_loaded -> returns a boolean and forces us to
update the roleinfo data.
Updaated the module name in the comments, fixed comment references
Removed commented lines and fixed minor comments
Some unused methods were commented for testing, they shouldn't appear in
upstream
fixed references to the repository tool in comments.
Fixed whitespace errors between methods.
Added a comment block describing the project.cfg file
updated method descriptions in the comment blocks