Added ZString.c_str() function; Added MStrin operator=.

MString.cpp

@@ -47,6 +47,15 @@ namespace coreutils {
       if(data)
 	free(data);
    }
+
+   MString& MString::operator=(coreutils::ZString& value) {
+      if(*this == value)
+	return *this;
+      int len = length;
+      setSize(length + value.getLength());
+      memcpy(data + len, value.getData(), value.getLength());
+      return *this;            
+   }
    
    int MString::write(char ch) {
       setSize(length + 1);