Fixed #35.

The first plan for this fix was just to workaround not to pass
`NSNotFound` to `selectTabAtIndex:`, but I found `selectTabAtIndex:`
doesn't take out-of-bounds arguments into account. In private methods
like `viewControllerAtIndex:` handles invalid arguments collectly so I
decided follow this way. Notice that `selectTabAtIndex:` is a public
method which behaviour will be changed by this fix.
Fixed a tab scrolling bug.

Before this, sometimes tab won't follow by scrolling when content view
is paged by swipe gestures. This was caused when `animatingToTab`
property is set to `YES` and never gets back to `NO` again. So I just
worked around this by adding a completion handler to cases that was not
handling this property. I'm not sure that was intentional or not, but
for me this fix works well.
Lowered required iOS SDK version to 6.0

It was set to 6.1 but I don't think there's anything that requires
strictly SDK 6.1.
Lowered target iOS version of xcodeproj as well.
Added DS_Store to gitignore.