Definition of units.bar is off by a factor of 1000

(first time using GitHub and nowhere near proficient enough to fork and change things myself, so I hope this is in the right place!)

The definition of the bar (astropy/units/si.py:151)

```python
def_unit(['bar'], 100 * Pa, namespace=_ns,
doc="bar: pressure")
```

is off by a factor of 1000. It should be something like `1e5 * Pa` or `100 * kPa`.