Parse rich text shared strings correctly

Hi,

I experienced some problems with complex Excel sheets with rich text strings.
In the current version, you only support plain text shared strings. When parsing rich text strings, the shared string dictionary is corrupted.

According to the official documentation regarding shared strings (http://msdn.microsoft.com/en-us/library/office/gg278314%28v=office.15%29.aspx), I implemented changes that also supports rich text strings (although ignoring their formatting, as this is useless in the context of creek).

Although my changes work for Excel documents I tested against (I also included a spec which checks against the shared string example in the documentation), I'd appreciate a review of my changes.

Thank you very much in advance!