Feature/smart picker placement

Issue #364 - Smart picker placement

Added an option, placement, that accepts "top/bottom" and "left/right".
For example, for picker placement at the top right of the input, use
```
.datepicker({placement: "top right"})
```

For auto picker positioning use ```.datepicker({placement: "auto"})```

A preferred position can be specified in addition to "auto", as long as "auto" is the first word. ```.datepicker({placement: "auto top right"})``` will try to place the picker with priority given to the top and right.

If top/bottom or left/right are not specified, the vertical/horizontal position will be centered relative to the input field. If both are not specified the picker is centered on the input field.