PARQUET-63: Enable dictionary encoding for FIXED.

This uses the existing dictionary support introduced for int96. Encoding
and ParquetProperties have been updated to use the dictionary supporting
classes, when requested for write or present during read. This also
fixes a bug in the fixed dictionary values writer, where the length was
hard-coded for int96, 12 bytes.

Author: Ryan Blue <rblue@cloudera.com>

Closes #30 from rdblue/PARQUET-63-add-fixed-dictionary-support and squashes the following commits:

bc34a34 [Ryan Blue] PARQUET-63: Enable dictionary encoding for FIXED.