Add ENUM ConvertedType

ConvertedType was missing the ENUM type and was not in parity with OriginalType.

I realized this after finding that annotations were lost after writing out a Parquet file and then reading it and saw that the converted_type field of SchemaElements were never set. This resulted in an exception when I tried reading a Parquet file containing maps using Pig caused by an incorrect converted Pig schema where a Parquet map was converted into a Bag rather than a Map. I am working on a prototype fix for this issue and will send a parquet-mr pull request when it is done.