This PR fixes three issues I encountered using the SST cache feature due to errors parsing xl/sharedStrings.xml:

1. Truncated text where eg. B1 is Blank ---> was being parsed as B1 is Blank ---. The XML parser was parsing the > at the end of the string as a second XMLEvent which the code ignored. Fixed by using XMLEventReader.getElementText() instead of XmlEventReader.nextEvent().asCharacters(). Example input:

   <sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="1" uniqueCount="1">
     <si>
       <t>B1 is Blank ---&gt;</t>
     </si>
   </sst>

2. The issue with blank strings described in com.sun.xml.internal.stream.events.EndElementEvent cannot be cast to javax.xml.stream.events.Characters #100. This is fixed by using XMLEventReader.getElementText(). Example input:

   <sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="1" uniqueCount="1">
     <si>
       <t/>
     </si>
   </sst>

3. Strings with rich text formatting were getting dropped. Fixed by adding code to parse the <r><t>...</t></r> pattern (Rich Text Run). Note that the new code still drops the formatting. Example input:

   <sst xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" count="2" uniqueCount="2">
     <si>
       <r>
         <rPr>
           <b/>
           <sz val="10"/>
           <rFont val="Arial"/>
           <family val="2"/>
         </rPr>
         <t>shared styled string</t>
       </r>
     </si>
   </sst>