Created contextMenu service, migrated open/close functionality into serv...

...ice

Inspired by https://github.com/ianwalter/ng-context-menu/pull/4 takes a step further to ensure multiple context menus behave as you would expect when opening/closing.

Keeping menu item positioned absolute, updated item placement to be based off of event.offsetX/Y rather than pageX/Y.  Still off if menu trigger element has children with padding.

Still need to add ability to set a default z-index value and allow override.