Add parser for BigDecimal

I [need](https://github.com/rmetzger/stratosphere-sql/issues/17) BigDecimal support the Stratosphere SQL interface.
The pull request adds a `DecimalValue` to Stratosphere, including a parser for the CSVInputFormat and a little test.

I also added a `GenericCsvInputFormat.getFieldParser()` method to allow external configuration of the parser. I will also need this for the [planned `DateValue` type](https://github.com/rmetzger/stratosphere-sql/issues/16) to configure different Date parsers.