Bulletproofing the `RomeoAndJuliet` exercise from hacky solutions.

[hristog]

An example of a possible abuse can be found here: d7c34a7.

Note:

• Quite a bit more elegant bulletproofing could be achieved via employing metaclasses and descriptors (thus, avoiding the annoying copy-and-paste's to replicate functionality) - if this PR gets accepted, I'll create a further commit implementing the improved version.
• The module imports (i.e., referring to Exercises instead of Advanced) on your master branch are probably outdated.
• The strange whitespace edits/replacements in arbitrary locations - outside of the intended ones only - within the edited files my commits have been created automatically, just by the mere virtue of editing the files.