MUMUP-1386 : make the placeholder card relative to mouse instead of original location

## Before
![dragndropnotworking](https://cloud.githubusercontent.com/assets/3534544/5343886/bdee245a-7ecf-11e4-9e63-7762bf8dce1b.gif)


## After
![drag and drop works](https://cloud.githubusercontent.com/assets/3534544/5343846/83f2611c-7ecf-11e4-9fd2-5507d47da0fc.gif)