Restructure the source code

[kdidenko]

Move all extension's assets into corresponding directories group. E.g.: all JavaScript files must belong to ./scripts/ directory, as all images must be moved to ./images/, etc... New structure must be similar to:

+/tabsaver/
+--- /scripts/
|     ---- *.js
+--- /styles/
|     ---- *.css
+--- /images/
|     ---- *.jpg
|     ---- *.png
|--- dialog.html
|--- manifest.json
+--- README.md