BUG: Fix bug for input with int elements

I found the [same bug](https://github.com/QuantEcon/QuantEcon.jl/pull/23#issuecomment-64198872) as in the Julia version. For an input matrix with integer elements, `gth_solve` returns a wrong answer:

```python
>>> P = [[-3, 3], [4, -4]]  # Transition rate matrix
>>> gth_solve(P)
array([ 0.5,  0.5])
```

The correct answer is [4/7, 3/7].
(My choice of the instance was not clever so that the test failed to detect this bug...)

The current code now converts the input to float.