Set contentMode to UIViewContentModeRedraw

As stated in the UIView Class Reference: “Changing the frame rectangle
automatically redisplays the receiver without invoking the drawRect:
method. If you want the drawRect: method invoked when the frame rectangle
changes, set the contentMode property to UIViewContentModeRedraw.”
Add sectionCount method

Refactor code by centralizing how the section count is retrieved. Also
add check for dividing by zero which would have resulted in segmentWidth
being a NaN.
Update segments on layoutSubviews to support auto-layout

layoutSubviews will get called when constraints are being applied. This
ensures that the frame of the scrollview and the segments will get
updated accordingly.

Fixes #22
Merge branch 'feature/autolayout-support' of https://github.com/lixar/HMSegmentedControl into lixar-feature/autolayout-support

Conflicts:
	HMSegmentedControl/HMSegmentedControl.m