Add support for plugin icons. As requested in YahnisElsts#59

Put the icons in the new `icons` subdirectory.

Remove another ?> tag and trailing whitespace

Fix occasional double slashes in download URLs.

In some environments, the guessServerUrl() method could return an invalid URL containing two forward slashes "//" instead of one. This caused update downloads to fail in WP 4.6. See bug report:
YahnisElsts/plugin-update-checker#66

The bug was in the *nix-like branch in guessServerUrl() that always added a forward slash to the path, even if there was one there already. Fixed by simplifying the method and only adding the slash only when necessary.

Merge changes from master

Merge pull request YahnisElsts#11 from jrfnl/Fix-serialization-issues

Base64-encode cached data to fix serialization issues