Enable menu toggle when sliding is disabled

mEnabled is used to check if mContent receives dragging events, shoud not be used to stop actions. 
if mEnabled is checked in scrollTo(int,int) method, side menu can still be called out by SlidingMenu.showMenu(), but the menu would be blank.
Merge pull request #335 from liruqi/patch-2

Enable menu toggle when sliding is disabled
Made mode left and mode right, flags so they can be properly combined using bitwise or.
Added xml attribute to set the secondary view.