Gemfile should be prettier

[shanab]

I suggest we group gems semantically. My proposal would be to group them into the following groups:

1. source
2. rails gem + database gem
3. backend gems
4. frontend gems
5. development + test group
6. development gems only (if any)
7. test gems only (if any)

There should be a blank line between each pair of groups. Also I suggest we order gems with a specific group alphabetically (backend/frontend gems can grow in size if it's a big project).