Check if user already exists in contacts list

Merge pull request #8

Check if user already exists in contacts list

Premium subscription advantages changes

• Remove contacts limit for non-premium users, make it paid required.
• Change premium info text.
• Make new contact creation paid required.

Catch errors, make configuration easier

• Add commands.json file in the root of the project to configure bot's commands.
• Change /pay command to /send due to conflicts with other bots.
• Catch error when user has not enough balance in order to commit a transfer.

Refactoring services

Merge pull request #5

Refactoring and cleanup code

Economics system improvements

• Add ability to earn duck-duck coins.
• Make /yemek command paid.