Iss33

Fix for #33 

*Summary*
Since `FileSplit` boundaries are determined without looking at the data, the record readers need to be modified to find the next valid record boundary regardless of where the split's `start` byte offset is.

*Changes*
* Update `EnclosedJsonInputFormat` to return false for `isSplitable()`
* Update `UnenclosedJsonRecordReader` to respect `start` and `length` byte offsets defined in the `FileSplit`
 * Added function `moveToRecordStart()` which will find the next record in the document given an arbitrary seek point
* Added test cases for UnenclosedJsonRecordReader