Add support to BigDecimal data type

When using BigDecimal, Sugar always returns null. After this update, Sugar will be able to cast the String value stored in database to a new BigDecimal object, which takes a String as constructor parameter.
You're missing an import of `java.math.BigDecimal`. Also, the second half of this is missing -- we should explicitly set up the database columns to `TEXT` for `BigDecimal`, and explicitly serialize `BigDecimal` objects to `String`'s before adding those records in.