refs #37 - To suppress travis-ci perl-5.18 perl-5.20 failures. orz...
XHF format docs, an api change and a bug fix.

* XHF now has eBNF definition. (I hope it is correct)
* API change - ": bar" (name-less field) is obsolated. please use "- bar" instead.
* fix `->read()` returning `+{}` even after eof