Test case for bytes/unicode issue with Entrez.epost under Python 3

Reported on the mailing list by Nicolas Joannin
http://lists.open-bio.org/pipermail/biopython/2013-January/008307.html
http://lists.open-bio.org/pipermail/biopython/2013-March/008430.html
Fix Python 3 bytes/unicode issue with Entrez.epost

Based on fix from Nicolas Joannin on the mailing list,
http://lists.open-bio.org/pipermail/biopython/2013-March/008419.html
Skip test_Entrez_online.py under Jython
Make Jython DTD parsing check specific (in case they fix this)