Using BPDisplaySettings again, re-add wrapper object BPAttributedStringConverted to keep public API intact

Here's another one for you guys :-) 
I figured that this repo is going to be the new main repo for iOS and since it has already removed the boost dependency I switched to it today (since I want to build using Xcode 5).

This pull request basically re-adds some of the fixes/additions made in the last week, that didn't make it here because of the visitor pattern.

 1. BPDisplaySettings are used/respected again
 2. Fix for random crash in renderSpanElement:withFont:attributes:toTarget: (text == nil) is re-applied
 3. Strikethrough works again
 4. Added a wrapper object BPAttributedStringConverter that internally uses the new visitor pattern, but keeps the public API intact (and makes the parser easier to use from outside)

I haven't done extensive testing yet, but I didn't encounter any issues so far.

What do you think?