add a feature, add a character, add url to README, remove useless code
speechBubbles/speechBubbles.py

* add a feature: Display particles at the center of the selection (absolute)
* add a character: Ћ
* add url to information about characters and emoticons (README.md)
* remove some unneccessary code
update label text
speechBubbles/speechBubbles.py

* update label text