Fix some issues to work better in scrolled pages

[rzucareli]

I fixed some issues in the module the improve the UX.

• Remove of the invisible div angular-bootstrap-contextmenu: Originally, when you open the context menu, the div was created and you have to click out of the menu to destroy both. The div blocks for a moment any iterate with the page, Removing it, the module close the context menu and do the any action the page. With that improve, the behavior of the context menu will be the same behavior of the default browser context menu.
• Fix the open position in scrolled pages: In scrolled pages, when you open the context menu in any position above the start position (scroll position = 0), the menu always open from the bottom to the top. Fixed this issue to considerate the scroll position in the calculation.
• Align the position of nested menu with their parent when it opens from the bottom to the top: When the nested menu will open from the bottom to the top adds the parent pixels height in the top position to align the nested menu to its parent.
• Close on scroll: Close the context menu when the page scroll down

[juanmarcus93]

Thank you, @rzucareli Nice improvements

[josetaira]

Looks good. Thanks!

[josetaira]

Actually there was a bug that you can no longer click outside to close the menu. Fixed that with d8ca2d4