Use PEP-426 environment markers

Any attempts to use platform_python_implementation or
implementation_name result in an "Invalid environment marker" error.

Any attempts to use >= to compare python_version result in an "Invalid
environment marker" error.

Augh documentation for all of this is painfully suboptimal.

Should fix #36

log ConflictError details, fixes #29

Update issue #22

Incorporate feedback and make the tests run under Python3. Notably, the Grand Exception Unification in Python 3.3 means that IOError is a synonym for OSError in those versions, but not before; deal with this by catching both types and throwing OSError in the doctests (because that's how it prints). Fix #21. Fix #22.

Fix #21, FileStorage: when disk space runs out while packing, do not leave the ``.pack`` file around

fixed ResolveableWhenStateDoesNotChange, changed show_tryToResolveConflict_log_output to check both cases

Support PyPy. 

This is a performance optimization for PyPy (profiling shows many recursive calls to Persistent.__getattribute__, and recursive calls seem to trouble the PyPy JIT; this eliminates many of them when loading objects):

"Transaction",                 AFTER       BEFORE
"Add 3000 Objects",             20205      18505
"Update 3000 Objects",          24191      18807
"Read 3000 Warm Objects",       28920      26671
"Read 3000 Cold Objects",       28745      27487
"Read 3000 Hot Objects",        29563      27487
"Read 3000 Steamin' Objects", 1033758     916587

Go back to assuming that cache.lru_items and cache.cache_data are in sync. Otherwise we have corruption in the cache and, under PyPy, a CFFI-level memory leak; we want tests to fail in that case.

Fix test_new_ghost_w_persistent_class under PyPy3, failing due to a bytes/unicode issue. It seems the cPickleCache under Python 3 accepts unicode objects as the oid, but the pure-Python version doesn't.

Rollback some changes now that Python persistent handles registering objects during __setstate__ the same way as C persistent.

Fix unpickling POSError subclasses under PyPy. This fixes a bunch of '<Unprintable POSKeyError>' tests in ZEO.