add docs to .gitignore
build doc on travis

Closes #14
document interp.mli
document interp_types
make doc: restore to correct directory
document parser
Document printer
document syntax
Merge branch 'doc'

* doc:
  document syntax
  Document printer
  document parser
  make doc: restore to correct directory
  document interp_types
  document interp.mli
  build doc on travis
  add docs to .gitignore
Link to API documentation

Closes #13