Binomial coefficients are not expanded by default

Initial fix for issue 3544. Now,
```
>>> b = binomial(n, 8)

>>> print b
>>> binomial(n, 8)

>>> b.expand(func=True)
>>> n**8/40320 - n**7/1440 + 23*n**6/2880 - 7*n**5/144 + 967*n**4/5760 - 469*n**3/1440 + 363*n**2/1120 - n/8

>>> expand_func(b)
>>> n*(n - 7)*(n - 6)*(n - 5)*(n - 4)*(n - 3)*(n - 2)*(n - 1)/40320
```

Let me know if more changes are required.
I expect many tests to fail because of this change. I'll update them later.