Support custom content view scale & offset when show menu

Added contentViewScaleValue, contentViewInLandscapeOffsetCenterX &
contentViewInPortraitOffsetCenterX to support defining custom scale &
offset of content view when show the menu.

In iPad landscape case, the content view will scaled to very small & the
menu's width is too wide.
Merge pull request #48 from iOSLabs/master

Support custom content view scale & offset when show menu