discodop grammar: fix --bitpar option
add treetransform for fanout markers
cache result of corpus.trees() and corpus.sents()
require unicode switch argument.
properly look up rules with rule number.

(only disambiguation of bitpar output and doprerank is affected)

- add grammar.revmap, to translate rule number to index for
  grammar.bylhs[][...].
- in grammar.rulenos[], store rule keys as strings instead of tuples.
- add regex argument to grammar.getrulemapping.
fix bug when heapify is called with empty heap.
Refactor heap, improve nwaymerge()

- switch names of siftup / siftdown to agree with Python heapq module.
- port merge() of Python heapq module.
update README; thanks @heatherleaf

- add gcc and --prefix= option to Mac OS X instructions.
- add 'make inplace' so that discodop modules are importable from within
  the main disco-dop directory.
refactor assert to proper runtime exceptions

- refactor assert to proper runtime exceptions
  (python -O disables asserts, so assert should only be used for
  non-essential testing purposes).
- comment formatting (always a space after #)