Demo classes updated

demo classes up to date in Medium Candy project
Revert "Demo classes updated"

This reverts commit 792577b9f0822cc34731bb7be04c07ebba6c0a67.
Revert "Revert "Demo classes updated""

This reverts commit a8596525a22b4a915a5d401f869ac8bf9e3dfc86.
updating project

persistence class update
UrlShortener initial version
URLPing

implemented private method that pings a given URL
new methods - with inheritance issues

inheritance issues
Merge remote-tracking branch 'origin/master' into developer
method description - inheritance issues solved

just method description.
core is up to date, so there are no longer inheritance issues in our
code.