Support for absolute left/right placement

I added new options for the `placement` setting: `absleft` and `absright`.  It positions the dropdown absolutely to the very left or the very right.  This helps with narrow UI (mobile) support in a "dumb" fashion.

### `placement: 'absleft'`

![image](https://cloud.githubusercontent.com/assets/324507/3556977/5d002ab4-092a-11e4-9832-bcb3f71f7640.png)

### `placement: 'absright'`

![image](https://cloud.githubusercontent.com/assets/324507/3556981/68b2506c-092a-11e4-88a2-be9baee1fa6c.png)

You can mix and match with `top`

### `placement: 'top|absleft'`

![image](https://cloud.githubusercontent.com/assets/324507/3557030/f0fe6e42-092a-11e4-8445-234ae6b91151.png)


### `placement: 'top|absright'` ( `placement: 'topabsright'` is also acceptable)

![image](https://cloud.githubusercontent.com/assets/324507/3557004/a1bc3738-092a-11e4-947d-7a282a6453d3.png)


@catherinek's solution dynamic placement is handy, but unfortunately is still susceptible to cutoffs, which is why I opted for absolute positioning.

![image](https://cloud.githubusercontent.com/assets/324507/3556938/0101338e-092a-11e4-9263-47d839deb3b4.png)