update .gitignore

turn `default_fantasy.jpg` untrackable, 'cause we may change it overtime, and it' s an image...
refactors, especailly for text.
update readme metadata, etc.
refactors.

1. file name.
2. menu item text color.
adjust timeline' s line.

1. specify the avatar' s width and height, ensuring the size on all screen size.
3. remove `just now` time indicator, use `s` instead.