Chrome Storage implementation in place of local storage and option to enable/disable sync

In order to sync the user credentials like (api_key, api_username, api_url, cache_content) we need to use the chrome.storage.sync feature instead of local storage.

Also, with chrome.storage.local/sync boolean variables can be stored as is without converting them to strings and thus eliminating the need to convert them back to booleans. 

Following commit implements the same and fixes some typos in the documentation along with some fresh documentation. 

Addresses modified #249 as per discussion.