Fix parsing of XML entities and xs:bool

[HelloOO7]

This PR fixes 2 issues that I've encountered when parsing VDV 301 schemas:

• xsdcpp only unescaped XML entities present within attributes, not element text contents. This prevented correct parsing of data such as:

  <StopName>
      <Value>Černý Most &lt;icon type="c_UndergroundB">[B]&lt;/icon></Value>
      <Language>cs</Language>
  </StopName>

  The PR fixes this by calling unescapeString when parsing elements.
• XSD allows the type xs:bool to be expressed both as text values of true/false and as integer literals 1/0. However, xsdcpp only supported the former (using std::boolalpha). My fix will attempt to use std::boolalpha, and if that fails, retry with std::noboolalpha before declaring the boolean as invalid.