Add Network settings, support multisite installations

1. Resolved #1 - Checks multisite flag, if set, networks settings override
   local settings.
2. Resolved #9 - Replaced get_settings with get_option, Replaced
   capability(was set to '8') parameter in 'add_submenu_page' and
   'add_options_page' functions with variable name(i.e. name of the
   capability).
3. Partly worked on Brand Plugin feature
	- Updated error messages
	- Updated title of the plugin in menu.
Merge pull request #2 from TriptiSharma/dev

Add Network settings, support multisite installations