Add topics to beer-song exercise

Add topics for space-age

Adding the list of topics for the phone-number problem

Add topics to pangram exercise

Add topics to twelve-days exercise

Adding the prerequisites to the CONTRIBUTING.md file

Add topics to raindrops exercise

* issue fixed

* minor bug fixed

* issue fixed #789

* minor bug fixed

Add Isogram topics

saddle-points: implement toString for matrix coordinate

Referencing #788

conditionals: finding if a char is repeated
loops: looping through a string's characters
parsing: parsing a string for repeated characters
strings: self-explanatory :)

referencing #788

conditionals: finding if a char is repeated
loops: looping through a string's characters
parsing: parsing a string for repeated characters
strings: self-explanatory :)

Fixes #782

Added topics for Bob exercise

#753 Improve test names - difference-of-squares

gigasecond: add topics

This is to improve the error message when comparison fails.

hamming: add topics

two-fer: add topics

Fix mistake in contributing guide

Add topics for hello-world

Revert "Update to Gradle 4.1"

This reverts commit a54c18e52ed0e37b2e808249c61c1329d39dd517.

Restructure exercise directories