Better random entry picker.

Should provide a more uniform distribution.
Refactoring and bug fix.

- Default to an empty object when retrieving user preferenes from
  storage, otherwise it fails on fresh installations;
- Split user preferences assembly into 2 separate functions;
- Remove unused `chrome.runtime` reference;
- Improve comments.
Fit into 80 characters width.
Tentative fix for hanging questions on uninstall.

Contentscript now tries sending a message to the background page every
time the contentscript's page becomes visible. If the message fails, the
question is removed. Obviously, this is not a perfect solution which
fails for pages that are already visible (e.g. in another window), but
there's apparently no API uninstall event to hook to...
Fix non-applied `outline:none` of nav elements.
Refactor.

- Tentative fix for a failing focus on the answer field;
- Reduce nesting in message handler by using `return` instead of `else`
  blocks;
- Less variable declarations in `removeQuestion` function;
- Rename `resetAnswerElement` to a more semantic `setupAnswerElement`;
- Outsource anonymous new question initialization function to a named
  one;
- Improve comments and variable names.
Remove screenshots.
Bump version to 0.6.3
Fit the installation instructions to releases.