Allow the user to specify whether the TITokenFieldView should be scrollable.

If the user allows scrolling, we still disable scrolling internally while the results table
is showing - if the user allows scrolling (which is the default value) then there should
be *no change in behaviour* in this commit.

If the user disallows scrolling, then the entire view should *never* be scrollable. This
introduces the ability to specify add a scrollview to the contentView, meaning that the
To: box always appears on screen, and doesn't scroll away with the content, as is the
current behaviour.
Merge pull request #3 from sadlerjw/allow-scroll-disable

Allow the user to specify whether the TITokenFieldView should be scrollable
Put layout-related code in layoutSubviews instead of setFrame

When the view is part of an autolayouted view hierarchy and its size is updated
via a change in constraints, setFrame is not called, but layoutSubviews is. So to
properly support autolayout, we need to do these calculations as part of layoutSubviews.
Fix calculations for _contentView's height

The content view's height depends on _tokenField's frame, therefore we have to
calculate _tokenField's frame first.
Don't call layoutSubviews directly; always use setNeedsLayout instead.
Allow TITokenFieldView to manage the frame of its own content view.

This enables the use case where the TITokenField is pinned to the top of the screen
and then contents of contentView are allowed to scroll independently. Usually if
managesContentViewFrame is set to YES, you'll also want to set scrollEnabled
on the TITokenFieldView to NO.
Merge pull request #2 from sadlerjw/autolayout-fixes

Autolayout fixes