Fix API bug in Stream.integrate

The API for integrate on a trace allows arguments to be passed into scipy integrate (example of initial value in cumtrapz), but this was missing on the stream implementation.  
PEP8: The two lines above should be:

```python
    tr.integrate(type='cumtrapz', initial=0)
st2.integrate(type='cumtrapz', initial=0)
```
Here there is no space around the `=` sign, since it is in a function argument.