flexible height / maxHeight option

height and  maxHeight option can pass a callback function or string
'auto',
for recalculating height/maxHeight of modal-body  everytime window's
size changed.

1. with default maxHeight = null
![nserver-bootstrap-06](https://f.cloud.github.com/assets/822889/132508/ec2b736c-7086-11e2-91d1-051cf2a40d24.png)

2. with maxHeight = 'auto'
![nserver-bootstrap-07](https://f.cloud.github.com/assets/822889/132515/1347dfbc-7087-11e2-82df-25a136e5dc5d.png)