Add xfail decorators for tests involving libfuturize with ``input()`` and Py3's lib2to3
Mark test_raise_ as xfail on Py3 for now
Fix some more failing tests and disable some that won't work on Py2
Some Python 2.6 fixes
Fix more tests on Python 2.6
Remove past.tests.base

- We use the one in ``future`` since the source trees for the two packages are
  intertwined anyway.
More test fixes and bypasses. Now all tests pass on Python 3.3, 2.7, 2.6