add initWithCode and init AXStretchableHeaderTabViewController's view…

[homekuanyi]

… in viewDidLoad instead of initWithNibName; add switchToIndex to switch to other child view controller

i hope u would consider my changes:

1. we can draw a UIViewController and set it to AXStretchableHeaderTabViewController, and IB can init it correctly now. since ios will init view controller's view automatically, it's better to do view init work in viewDidLoad instead of initWithNibName.
2. add switchToIndex function. i call self.selectedIndex = 1 to select a tab, but it doesn't work correctly.

[homekuanyi]

i have test it in ios 8.4