Merge s2t branch

This adds a new string-to-tree decoder, which can be enabled with the -s2t
option.  It's intended to be faster and simpler than the generic chart
decoder, and is designed to support lattice input (still WIP).  For a en-de
system trained on WMT14 data, it's approximately 40% faster in practice.

For background information, see the decoding section of the EMNLP tutorial
on syntax-based MT:

  http://www.emnlp2014.org/tutorials/5_notes.pdf

Some features are not implemented yet, including support for internal tree
structure and soft source-syntactic constraints.