Adding option to remove shadow from content view

showShadowOnContentView can be set with a BOOL to disable shadow
Merge branch 'remove_shadow' of https://github.com/hashrocketeer/NVSlideMenuController into hashrocketeer-remove_shadow
Update shadow show/hide logic to work after content view did load.
Update CHANGELOG & README.
Merge branch 'hashrocketeer-remove_shadow' into develop