Remove our huge checklist in the file

We have github issues and CHANGELOG.md for that
PEP8
Revise file_history.py a bit

- Add some TODO and TOCHECK comments
- Fix fallback to default timestamp format
- use `with` statements where possible
- indent timestamp in the quick panel a bit
Update relative timestamp generation

Removes plural s's.
Now only shows "1 year" instead of "1 year, 24 minutes" because it is
limited to 2 magnitudes starting from the first non-zero. More likely to
trigger once months and weeks are added.
Include months and weeks in rel calculation
Use dedicated keymaps for each OS and fix indents

Also update the Main menu to point to the new files, using the $platform
variable.

Closes #21.
Revise and wrap install.txt
Rename a few timestap-related settings
Update README to the recent changes

- Added new screenshots
- Removed manual installation because people should either use Package
  Control or know what they are doing
- General typo fixes and rewording
Slightly rename command palette commands
Rename install.txt to install.md
Prepare for 1.5.0 release and update changelog