Fixes to tab storage

There were some issues in the tab handling and storage. This release fixes the following bugs:

• Tabs could not be deleted (#24)
• When loading tabs for the first time after installing the extension, they did not actually load
• Old storage might not be deleted (#21)

Bugfix release for severe issue

A change in Firefox caused the extension to not load anymore. This release fixes the issue.

The tab groups are coming

Finally the long-awaited tab groups feature is coming to SaveTabs.