Modified argv() to return the original "char **" instead of "char const **".

According to C99, section 5.1.2.2.1 (Program startup):
"The parameters argc and argv and the strings pointed to
 by the argv array shall be modifiable by the program, and
 retain their last-stored values between program startup and
 program termination."
And it makes using boost::program_options much easier :)