PARQUET-107: Add option to disable summary metadata.

This adds an option to the commitJob phase of the MR OutputCommitter,
parquet.enable.summary-metadata (default true), that can be used to
disable the summary metadata files generated from the footers of all of
the files produced. This enables more control over when those summary
files are produced and makes it possible to rename MR outputs and then
generate the summaries.

Author: Ryan Blue <rblue@cloudera.com>

Closes #68 from rdblue/PARQUET-107-add-summary-metadata-option and squashes the following commits:

261e5e4 [Ryan Blue] PARQUET-107: Add option to disable summary metadata.