get_pointing_chain() now handles coreference chains w/ multiple antecedents
- cf. Issue #40
- chains with multiple antecedents increase the number of chains, e.g.
  a chain A -> B -> C&D will be turned into two chains:
  A -> B -> C and A -> B -> D
mmax2: added comments concerning double antecedent handling, removed dead code
mmax2: added span2text()
mmax2: renamed span2tokens() to spanstring2tokens() and span2text() to spanstring2text()
updated make merge-only (now includes mmax2)