Update ng-context-menu.js

The position fixed allows to place the div content menu anywhere.
I take the menuElement only on click, so the dom/html is ready, I use your directive in another directive and with your code it doesn't find the menuElement because your directive is executing before my directive is completed. Do a test with your directive into another.
Merge pull request #4 from coolweb/patch-1

I'm going to merge this but I'm going to have to do a little cleanup before I make a new release tag. One thing I'm going to do is remove ```menuElement.css('position', 'fixed');``` completely since it should be handled by CSS anyway.
Merge cleanup, Adding CONTRIBUTING.md
Merge branch 'master' of https://github.com/ianwalter/ng-context-menu into issue4/service

Conflicts:
	src/ng-context-menu.js
Adding contextMenu service, updates for style guide