Bind Buffer Variables as binary values (with Native implementation also)

Avoids having to encode blobs in queries.

This is a reopen of issue #92 by @nraynaud

I've updated the original javascript-only PR to work with the current version of node-postgres, as well as implement changes in the native driver as well.

The native approach was to simply encode any buffer into the bytea hex format as suggested at http://www.postgresql.org/docs/9.0/static/datatype-binary.html