Added extra security checks on XSSAuditor.cpp

Added extra security checks, to protect xss auditor against "PHP array injection" and "PHP array-like injection" attacks. The results of this work have been presented in the paper entitled: Anastasios Stasinopoulos, Christoforos Ntantogian and Christos Xenakis “Bypassing XSS Auditor: Taking advantage of bad-written PHP code”, IEEE International Symposium on Signal Processing and Information Technology (ISSPIT 2014), December 15-17, 2014 - Jaypee Institute of Information Technology, Noida, India