Evaluating limits when substuting an expression with infinity. 

Using subs with oo leads to wrong results.
For example 
```
>>> (exp(x)/exp(x*x)).subs(x,oo)
nan

```
https://code.google.com/p/sympy/issues/detail?id=1769#c9

This attempts to fix this by adding a special case in the _subs function to evaluate limit when expression being substituted with is infinity.