Fix: Use backgroundColor of superview for centerContainerView

Allows to use color set in storyboard for background color of all subviews.
There is no way to do this from library user code, he is stuck with white color.

Biggest problem (use case): controller inside pager needs to be set to clearColor,
because we use window background image pattern.
add gitignore file
Merge branch 'pr/10'
修改 XHTwitterPaggingViewer.podspec