Fix for Issue #8, pkt.rewrite_field

Fix for Issue #8, pkt.rewrite_field

* This was unintentionally removed in the last cleanup, adding it back.
* Also adding support for $ip.dst = XXX style syntax
* This is an ugly commit, please bear with it until I figure out the
  structure of the language, especially w.r.t. proto:: type fields.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>