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
Added functionality

- Added ability to have dynamic segment widths (great when you have
scrolling enabled)
- Made segment control type public, to be used in combination with
storyboards (when you don’t need to use initWith…)
- Added ability to disable/enable drag when the control can be scrolled
(sometimes needed for user experience)
- Added ability to disable touch altogether (needed when you have
animated pages coupled with the segmented control)
- Used @khoogheem code to add both text & images to a segment. Modified
it to use with dynamic segment widths and put the image to the left of
the text
Merge pull request #26 from Thermometer/master

Dynamic segment widths, storyboards fix, text with images in segment
Merge branch 'feature/autolayout-support' of https://github.com/lixar/HMSegmentedControl into lixar-feature/autolayout-support

Conflicts:
	HMSegmentedControl/HMSegmentedControl.m