-DJSONCPP_USE_SECURE_MEMORY=1 for cmake

Add allocator.h to amalgamated header
Test JSONCPP_USE_SECURE_MEMORY in Travis
diff --git a/include/json/value.h b/include/json/value.h
index 5948935..20747fe 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -336,6 +336,9 @@
   int compare(const Value& other) const;
 
   const char* asCString() const; ///< Embedded zeroes could cause you trouble!
+#if JSONCPP_USING_SECURE_MEMORY
+  unsigned getCStringLength() const; //Allows you to understand the length of the CString
+#endif
   JSONCPP_STRING asString() const; ///< Embedded zeroes are possible.
   /** Get raw char* of string-value.
    *  \return false if !string. (Seg-fault if str or end are NULL.)