Added new feature to show sublime uptime helps you to know how long you working

Add: Control clock update interval from setting file
Add: setting to display clock only when there is a file in view
     or display always like before
Add: Show sublime uptime helps you to know how long you working
Fixed: AttributeError: 'NoneType' object has no attribute 'active_view'
Which was caused when there was no file in view
Fixed: missing item in documentation
Merge pull request #2 from s-sam/master

Hi, I did some modifications to your plugin