Only use activeTab permission

[fregante]

The activeTab permission gives an extension temporary access to the currently active tab when the user invokes the extension - for example by clicking its browser action. Access to the tab lasts until the tab is navigated or closed.

This serves as an alternative for many uses of <all_urls>, but displays no warning message during installation:

Without activeTab	With activeTab

[riosjuan]

Good suggestion! I did some research and in order make full use of the activeTab permission it requires some refactoring to remove the content_scripts.