Fix #51: DataLossWarning hiding real exceptions in parser.

Previously we stopped parser tests once DataLossWarning was raised; we
now run let the parser run to completion before checking for raised
warnings. This hid several cases of where real exceptions were raised
after DataLossWarning was raised. This commit reveals these real
exceptions in the testsuite and fixes current failure.

Similar fixes are needed for the other tests.
Add release date for 1.0b1.
Move default_etree to utils.py and use it for the treewalker too.
Add pypy to tox by default.

It's quite probable fewer people will have PyPy installed, but I
doubt many will have four versions of CPython installed either.
Fix release version.
Clarify variable names in test_parser.py.
Bump version to 1.0b2.
Fix the two cases of us using old-style classes on Python 2.