ENH  adding metadata argument to DataFrame.to_parquet
#### Code Sample, a copy-pastable example if possible

Please comsider merging

https://github.com/pandas-dev/pandas/compare/master...JacekPliszka:master



#### Problem description

Currently pandas can not add custom metadata to parquet file.

This patch add metadata argument to DataFrame.to_parquet that allows for that.
Warning is issued when pandas key is present in the dictionary passed.

