Fix parsing from std::cin (tellg/seekg are not working for it).

[SergiusTheBest]

Hi there,

I fixed parsing from std::cin. The problem was in tellg/seekg that break std::cin (and it looks they are not required at all, so I removed them).

You can reproduce the issue with the following sample:

#include "jsonxx.h"
#include <iostream>

using namespace std;
using namespace jsonxx;

int main()
{
    Object o;
    o.parse(cin);
    cout << o.get<Array>("ar").get<Number>(0) << endl;
    return 0;
}

Send this json to stdin:

{ "ar" : [1] }

[hjiang]

This change would break the test at jsonxx_test.cc:171 on macOS.

But it seems to work fine on Linux. Interesting.

[SergiusTheBest]

I checked on Windows and Linux. So it looks it depends on istream implementation.

[SergiusTheBest]

By the way, thanks for the good library. It's small and very easy to use. Nice work!