Check Internet connection before executing Async task, fixes #13

Throw Toast in case of exception
Added permission to check Internet connection
No Internet connection toast
Location and unit system shared preferences via ActionBar menu

Refresh method for weather
fixes #11, #12 and #14
Notify user with Toast if HTTP response / JSON parsing failed fixes #15
Refresh weather after shared preferences dialog, fixes #16
Merge pull request #17 from Predator7/shared-preferences

Shared Preferences