BilingualRule: include alignment

Instead of hard-coding a null return for getAlignment, include
alignment as a byte [] field. This adds one pointer of overhead
for every rule in a non-packed grammar regardless of whether
alignments are included or not.
HieroFormatReader: read optional alignment field
KBestExtractor: add rule alignment in %d output
Merge pull request #161 from jweese/rule_alignment

Rule alignment