README and parseIt cleanups to discourage multithreading
- update all references to multithreading to mention instability
- remove PETSc/TAO compilation instructions since cvlm is currently
  busted (and will be replaced by a version that doesn't require PETSc/TAO
  soon).
- fix clash between pretty printing and POS tag smoothing option parsing
  (both were 'P', smoothing is now 'p')
- minor cleanups in parseIt.C