Solved the bug with dates being added more than once in "days" array

    The current date was pushed to the "days" array every time the
    background.js was initialised, even if the current date was already
    there, in the array.

    The check for the key (current date) in the array was returning
    always false due to a stupid error. I was checking the key existance
    using the current date in a simple array. Now I'm checking if the
    last key of the array has the value of the current day.

    The "days" array will never have duplicate keys.
Solved the error on graphics when adding a new url in blacklist.

    After adding a new website url to blacklist, the bar chart was not
    loaded anymore. The error appeared because the function that is preparing the array
    for the barchart was looking for localStorage property with the name
    of the newly add url, which didn't exist if that website was not
    accessed.

    Every time a new url is added to the blacklist, a new property is inserted to
    localStorage with the name of that url.

    The function that is preparing the array for the bar chart will not
    brake when iterating over all the localStorage properties, thus the
    bar chart will be created even and will show also the newly added
    url evan if this url has no data attached to it.