possible to reuse the header view when sticky and when not

First thanks for your great library. I love it!
Now i did not really like the fact that when a header switch from sticky to normal (and reverse) you actually expect a "new" view to be created.
I prefer to actually reuse the the section header.  That commit allows that. 
Mostly the only changes are:
* make sure the headerView has no parent to prevent exception on addView
* when removing "sticky" header, add it back to the WrapperView if necessary

How you'll like it ;)