Storage - mechanism & plugin memory implementation

[BartoszKlonowski]

The goal of this issue is to implement the fully operational storage mechanism.

This mechanism should be able to:

• run locally, on a specific (or browser-specific) directory
• store various types of data (but mostly strings, long strings)
• be easy to read/write with simple API
• be light - the plugin needs to consume as few resources as possible.

Most probably the LocalStorage should be used, but it is necessary to check it against other browsers.

[BartoszKlonowski]

Other (more complex) option would be to use IndexedDB