Add new excel field type option for integers

Since excel stores all numbers as decimals, the importer can cause problems with applications that are expecting integer types. Options are: Plain/Date/Integer/Decimal, decided to make it consistent with case list sorting types. Plain and decimal treat things the same as always.

![screen shot 2013-10-25 at 1 12 52 pm](https://f.cloud.github.com/assets/152134/1409787/ee1d1b6e-3d98-11e3-8f91-970ddd79be47.png)