Skip logs on empty queue

This update makes logs more clear. When you don't have images to optimize - we won't write logs anymore.
Otherwise it just write empty log each interval and increase the DB size.

Site availability check

• New: Added notice if site is not available online.
• New: Added notice if PHP version is below 7.0.
• Bug fix: Dashboard image size statistics fatal error.

Requirements check

Added requirements check on Settings page to be sure plugin works fine.

Compatibility with JRI / Regen Thumbnails

• Compatibility with Just Responsive Image plugin v1.5+
• Compatibility with Regenerate Thumbnails plugin v3+

First stable release

Merge pull request #7 from justcoded/develop

v1.0 Added tries in optimization process, optimize only not optimize images