Fix validation errors in CSV imports.

#6054 

This PR addresses https://github.com/openscholar/openscholar/issues/6054#issuecomment-48781035

Output-
For the URL field in feeds we get:
![selection_351](https://cloud.githubusercontent.com/assets/4497748/3563762/a58b7384-0a50-11e4-83f6-82fca526f009.png)

For person, not multiple messages on missing first/last names:
![selection_349](https://cloud.githubusercontent.com/assets/4497748/3563766/bfcceaca-0a50-11e4-9c79-adbd8a32f4d3.png)
![selection_350](https://cloud.githubusercontent.com/assets/4497748/3563767/bfcf8f78-0a50-11e4-82f6-054bce85e9e3.png)

Also now the date for presentation can be empty.