PARQUET-67: mechanism to add extra metadata in the footer

this expands on the idea proposed by @wesleypeck in https://github.com/Parquet/parquet-mr/pull/185

Author: julien <julien@twitter.com>

Closes #32 from julienledem/extensible_metadata and squashes the following commits:

72e0a50 [julien] mechanism to add extra metadata in the footer