add support for native scroll and ion-side-menu

[mixersoft]

headerHeight was getting a value from the wrong element when using ion-side-menu

headerHeight = activeHeader.offsetHeight;

[MHabashy10]

Hi Everyone,

Thank you so much for this awesome scroll plugin, i'm having this issue with sidemenu, after scrolling back the subheader overlap the content, although my ion-content has the class 'has-subheader'
and i think this fix #26 solves the problem.

any plans to merge this pr soon?

Thank you again for this awesome feature
Regards

[MHabashy10]

anyone?
@djett41 @mixersoft